Electro-optical device with active matrix EL display

ABSTRACT

An electro-optical device for performing time division gray scale display and which is capable of arbitrarily setting the amount of time during which light is emitted by EL elements is provided. From among n sustain periods Ts 1 , . . . , Tsn, the brightness of light emitted by the EL elements during at least one sustain period is set to be always lower than the brightness of light emitted by the EL elements during the other sustain periods, and the sustain periods are extended by the amount that the brightness has dropped. In accordance with the above structure, the sustain periods can be extended by lowering the setting of the brightness of light emitted by the EL elements.

This application is a continuation of U.S. application Ser. No.10/435,968, filed on May 12, 2003 now U.S. Pat. No. 7,239,083, which isa continuation of U.S. application Ser. No. 09/692,713, filed on Oct.19, 2000 (now U.S. Pat. No. 6,587,086 issued Jul. 1, 2003).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an EL (electro-luminescence) display(an electro-optical device) formed by preparing an EL element on asubstrate. More particularly, the invention relates to an EL displayusing a semiconductor element (an element using a semiconductor thinfilm). Furthermore, the present invention relates to an electronicequipment in which the EL display is used in a display portion thereof.

2. Description of the Related Art

In recent years, technology for forming a TFT on a substrate has beenlargely improved, and an application development of the TFT to an activematrix type display device has been carried out. In particular, the TFTusing a polysilicon film has a higher electric field effect mobilitythan the TFT using a conventional amorphous silicon film, and therefore,the TFT may be operated at a high speed. Thus, the pixel control whichhas been conducted at a driver circuit outside of the substrate may beconducted at the driver circuit which is formed on the same substrate asthe pixel.

Such an active matrix type display device can, by preparing variouscircuits and elements on the same substrate, obtain various advantagessuch as a decrease in the manufacturing cost, a decrease in the size ofthe display device, an increase in the yield, and a decrease in thethroughput.

Further, research on the active matrix type EL display having an ELelement as a self-light-emitting device is becoming more and moreactive. The EL display is referred to as an organic EL display (OLED) oran organic light-emitting diode (OLED).

The EL display is a self-light-emitting type unlike a liquid crystaldisplay device. The EL element is constituted in such a manner that anEL layer is sandwiched between a pair of electrodes. However, the ELlayer normally has a lamination structure. Typically, the laminationstructure of a “positive hole transport layer/a luminous layer/anelectron transport layer” proposed by Tang et al. of the Eastman KodakCompany can be cited. This structure has a very high light-emittingefficiency, and this structure is adopted in almost all the EL displayswhich are currently subjected to research and development.

In addition, the structure may be such that on the pixel electrode, apositive hole injection layer/a positive hole transport layer/a luminouslayer/an electron transport layer, or a positive hole injection layer/apositive hole transport layer/a luminous layer/an electron transportlayer/an electron injection layer may be laminated in order.Phosphorescent dye or the like may be doped into the luminous layer.

In this specification, all the layers provided between the pixelelectrode and an opposite electrode are generally referred to as ELlayers. Consequently, the positive hole injection layer, the positivehole transport layer, the luminous layer, the electron transport layer,the electron injection layer and the like are all included in the ELlayers.

Then, a predetermined voltage is applied to the EL layer having theabove structure from the pair of the electrodes, so that a recombinationof carriers is generated in the luminous layer and light is emitted.Incidentally, in this specification, the fact that the EL element isemitted is described as the fact that the EL element is driven.Furthermore, in this specification, the light-emitting element formed ofthe anode, the EL layer and the cathode is referred to as an EL element.

An analog type driver method (analog drive) can be given as a method ofdriving an EL display. An analog drive EL display is explained usingFIGS. 18 and 19.

The structure of a pixel portion of the analog drive EL display is shownin FIG. 18. Y gate signal lines (G1 to Gy) for inputting gate signalsare connected to gate electrodes of switching TFTs 1801 of pixels. Oneof a source region and a drain region of the switching TFT 1801 of eachpixel is connected to x source signal lines (also referred to as datasignal lines) (S1 to Sx) for inputting analog video signals, and theother is connected to the gate electrode of an EL driver TFT 1804 ofeach pixel and to a capacitor 1808 of each pixel.

The source region and the drain region of the EL driver TFT 1804incorporated in each of the pixels is connected to the power sourcesupply lines (V1 to Vx) while the other is connected to the EL element1806. The potential of the power source supply lines (V1 to Vx) isreferred to as the potential of the power source. Note that, the powersource supply lines (V1 to Vx) is connected to a capacitor 1808incorporated in each of the pixels.

The EL element 1806 comprises an anode, a cathode and an EL layerprovided between the anode and the cathode. In the case where the anodeis connected to the source region or the drain region of the EL driverTFT 1804, namely, in the case where the anode is the pixel electrode,the cathode which is the opposite electrode is held at a constantpotential. On the contrary, in the case where the cathode is connectedto the source region or the drain region of the EL driver TFT 1804, thatis, in the case the cathode is the pixel electrode, the anode, which isan opposite electrode is held at a constant potential.

The opposite electrodes are normally maintained at a constant electricpotential, and in the present specification, the electric potential ofthe opposite electrodes is referred to as a steady-state electricpotential. Note that an power source for imparting the steady-stateelectric potential to the opposite electrodes is referred to as asteady-state power source. The electric potential difference between thesteady-state electric potential of the opposite electrodes and the powersource electric potential of the pixel electrodes is an EL drivervoltage, and the EL driver voltage is applied to the EL layers.

A timing chart for a case of driving the EL display by the analog methodis shown in FIG. 19. A period during which one gate signal line isselected is referred to as one line period (L). Further, a period untilselection of all the gate signal lines (G1 to Gy) is completedcorresponds to one frame period (F). There are y gate signal lines forthe case of the EL display of FIG. 18, and therefore y line periods (L1to Ly) are formed during one frame period.

Note that 60 or more frame periods are formed during one second in theEL display drive. In other words, 60 or more images are displayed duringone second. If the number of images displayed in one second becomes lessthan 60, then problems such as image flicker start to become visuallyconspicuous.

The number of line periods during one frame period increases as thenumber of gradations increases, and the driver circuit must operate at ahigh frequency.

First, electric power source supply lines (V1 to Vx) are maintained inan off-power source electric potential. Note that the off-power sourceelectric potential in an analog driver method is in a range in which theEL elements do not emit light and is the same strength as thesteady-state electric potential. Note also that the difference betweenthe off-power source electric potential and the steady-state electricpotential is referred to as an off EL driver voltage. Ideally, it ispreferable that the off EL driver voltage be 0 V, but it is acceptableprovided that it is such that EL elements 1806 do not emit light.

A gate signal is input to the gate signal line G1 in the first lineperiod (L1). An analog video signal is then input to the source signallines (S1 to Sx) in order. A switching TFT (1,1) is therefore in an Onstate (on), and consequently the analog video signal input to the sourcesignal line S1 is input to the gate electrode of an EL driver TFT (1,1),through the switching TFT (1,1).

The electric potential of the power source supply line V1 then changesfrom the off-power source electric potential to a saturation powersource electric potential. Note that, throughout this specification,saturation power source electric potential refers to an electricpotential having an electric potential difference with the steady-stateelectric potential to the extent that the EL element emits light. Notealso that this electric potential difference is referred to as asaturation power source voltage.

When the analog video signal is input to the gate electrode of the ELdriver TFT and one of the source region and the drain region ismaintained at the saturation power source electric potential, the otherbecomes the on-power source electric potential. Note that the differencebetween the on EL driver electric potential and the steady-stateelectric potential is referred to as an on EL driver voltage. Further,the on EL driver voltage and the off EL driver voltage are genericallyreferred to as an EL driver voltage throughout this specification.

The on driver voltage is then applied to the EL element and the pixelperforms display. The amount of electric current flowing in channelforming regions of the EL driver TFTs is controlled by the voltage sizeof the analog video signal input to the gate electrodes of the EL driverTFTs. The size of the on EL driver electric potential is thereforecontrolled by the analog video signal applied to the gate electrode ofthe EL driver TFT (1,1). Consequently, the size of the on EL drivervoltage applied to the EL element is also controlled by the analog videosignal applied to the gate electrode of the EL driver TFT (1,1).

Next, the analog video signal is similarly applied to the source signalline S2, and a switching TFT (2,1) turns on. The analog video signalinput to the source signal line S2 is therefore input to the gateelectrode of the EL driver TFT (2,1) through the switching TFT (2,1).

The EL driver TFT (2,1) is therefore placed in the On state. Theelectric potential of the power source supply line V2 then changes fromthe off-power source electric potential to the saturation power sourceelectric potential. The on driver voltage, whose size is controlled bythe analog video signal input to the gate electrode of the EL driver TFT(2,1), is therefore applied to the EL element, and the pixel performsdisplay.

By repeating the above operations and completing input of the analogvideo signal to the source signal lines (S1 to Sx), the first lineperiod (L1) is completed. The second line period (L2) begins next, andthe gate signal is input to the gate signal line G2. Then, similar tothe first line period (L1), the analog video signal is input to thesource signal lines (S1 to Sx) in order.

The analog video signal is input to the source signal line S1. Aswitching TFT (1,2) turns on, and therefore the analog video signalinput to the source signal line S1 is input to the gate electrode of anEL driver TFT (1,2) through the switching TFT (1,2).

The EL driver TFT (1,2) therefore turns on. The electric potential ofthe power source supply line V1 then changes from the off-power sourceelectric potential to the saturation power source electric potential.The on driver voltage, whose size is controlled by the analog videosignal applied to the gate electrode of the EL driver TFT (1,2), istherefore applied to the EL element, and the pixel performs display.

By repeating the above operations and completing input of the analogvideo signal to the source signal lines (S1 to Sx), the second lineperiod (L2) is completed. The third line period (L3) begins next, andthe gate signal is input to the gate signal line G3.

The above operations are then repeated in order, the gate signal iscompletely input to the gate signal lines (G1 to Gy), and all of theline periods (L1 to Ly) are completed. When all of the line periods (L1to Ly) are complete, one frame period is complete. All of the pixelsperform display during one frame period, forming one image.

Thus the amount of light emitted by the EL elements is controlled inaccordance with the analog video signal, and gray scale display isperformed by controlling the amount of light emitted. This method is adriver method referred to as the analog driver method, and gray scaledisplay is performed by changing the amplitude of the signal.

A detailed description of the state of controlling the amount ofelectric current supplied to the EL elements by the gate voltage of theEL driver TFTs is made using FIGS. 3A and 3B.

FIG. 3A is a graph showing the transistor characteristics of the ELdriver TFTs, and reference numeral 401 is referred to as an Id-Vgcharacteristic (also referred to as an Id-Vg curve). Id is a draincurrent, and Vg is a gate voltage here. The amount of electric currentflowing with respect to an arbitrary gate voltage can be found from thisgraph.

A region of the Id-Vg characteristic shown by a dotted line 402 isusually used in driving the EL elements. An enlarged diagram of theregion enclosed by the dotted line 402 is shown in FIG. 3B.

The shaded region in FIG. 3B is referred to as a sub-threshold region.In practice, this indicates a gate voltage in the vicinity of, or below,the threshold voltage (Vth), and in this region, the drain currentchanges exponentially with respect to the changes in the gate voltage.Electric current control is performed in accordance with the gatevoltage by using this region.

The switching TFT turns on, and the analog video signal input within thepixel becomes the gate voltage of the EL driver TFT. At this point, thegate voltage and the drain current vary linearly in accordance with theId-Vg characteristic shown in FIG. 3A. In other words, the drain regionelectric potential (the on EL driver electric potential) is determinedin correspondence with the voltage of the analog video signal input tothe gate electrode of the EL driver TFT, a predetermined drain currentflows in the EL element, and the EL element emits light with an emissionamount corresponding to the amount of electric current.

The amount of light emitted by the EL element is thus controlled inaccordance with the video signal, and gray scale display is performed bythe control of the amount of light emitted.

However, the above analog drive has a drawback in that it is extremelyweak with respect to variations in the TFT characteristics. For example,suppose that the Id-Vg characteristic of a switching TFT differs fromthat of the switching TFT of an adjacent pixel displaying the samegradation (a case of an overall positive or negative shift).

In this case, the drain current of each switching TFTs becomes differenton the degree of the variation, and a different gate voltage becomesapplied to the EL driver TFT of each pixels. In other words, a differentelectric current flows in each of the EL elements, and as a result, theamount of light emitted differs, and display of the same gradation cannot be performed.

Further, even supposing that equal gate voltages are applied to the ELdriver TFT of each pixels, if there is dispersion in the Id-Vgcharacteristic of the EL driver TFTs, then the same drain current cannotbe output. In addition, as is made clear from FIG. 3A, the region usedis one in which the drain current changes exponentially with respect tochanges in the gate voltage, and therefore even if the Id-Vgcharacteristic deviates by a slight amount, a situation can develop inwhich the amount of electric current output differs greatly even withequal gate voltages. If this occurs, then even if the same voltagesignals are input, the amount of light emitted by EL elements inadjacent pixels differs greatly due to a slight deviation in the Id-Vgcharacteristic.

In practice, there is a multiplier effect between the variations in theswitching TFTs and the EL driver TFTs, and therefore it becomesconditionally more difficult. The analog drive is thus extremelysensitive with respect to dispersion in TFT characteristics, and thisdisturbs the multi-colorization of conventional active matrix EL displaydevices.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems, andan object of the present invention is to provide an active matrix typeEL display device capable of performing clear multi-gray scale display.In addition, an object of the present invention is to provide a highperformance electronic device in which this type of active matrix ELdisplay device is loaded as a display.

The applicant of the present invention considers that the above analogdriver problems arise from the fact that the amount of electric currentflowing in the EL elements is controlled by using the sub-thresholdregion, in which the impact of dispersion in the Id-Vg characteristic iseasily felt because the drain current changes exponentially with respectto changes in the gate voltage.

Namely, when there are variations in the Id-Vg characteristic, there isexponential change in the drain current with respect to changes in thegate voltage in the sub-threshold region, and therefore differingelectric currents (drain currents) are output even if equal gatevoltages are applied. As a result, a defect develops in which desiredgradations are not obtained.

The applicant of the present invention therefore considers performingcontrol of the amount of light emitted by the EL elements by notperforming control of the electric current using the sub-thresholdregion, but mainly performing control of the amount of time that the ELelements emit light. In short, gray scale display is performed with thepresent invention by controlling the amount of light emitted by the ELelements with time. Performing gray scale display by controlling theamount of time which light is emitted by the EL elements is referred toas a time partitioning driver method (hereafter referred to as a digitaldrive). Note that gray scale display performed by the time partitioningdriver method is referred to as time division gray scale display.

By employing the above structure, variations in the amount of electriccurrent output when equal gate voltages are applied can be suppressedeven if there are small variations in the Id-Vg characteristic of theTFTs. It therefore becomes possible to eliminate the situation ofgreatly differing amounts of light emitted by the EL elements ofadjacent pixels due to variations in the Id-Vg characteristic, even ifthe same voltage signal is input.

The constitution of the present invention is shown below.

According to the present invention, there is provided an electro-opticaldevice comprising a plurality of EL elements and a plurality of pixelshaving the plurality of EL elements, wherein gray scale display isperformed by controlling the period during which the EL elements emitlight, and the brightness of the light emitted by the EL elements,during one frame period.

According to the present invention, there is provided an electro-opticaldevice comprising a plurality of EL elements and a plurality of pixelshaving the plurality of EL elements, wherein one frame period consistsof n subframe periods SF1, SF2, . . . , Sfn, the n subframe periods SF1,SF2, . . . , SFn have address periods Ta1, Ta2, . . . , Tan and sustainperiods Ts1, Ts2, . . . , Tsn, respectively, digital data signals in theaddress periods Tat, Ta2, . . . , Tan are input to all of the pluralityof pixels, the plurality of EL elements are selected to emit light ornot to emit light during the sustain periods Ts1, Ts2, . . . , Tsn inaccordance with the digital data signals, the brightness of the lightemitted by the EL elements during at least one sustain period Tsp (wherep is a natural number greater than or equal to 1, and less than or equalto n), among the sustain periods Ts1, Ts2, . . . , Tsn, is 1/m (where mis a positive number) of the brightness of the light emitted by the ELelements during an arbitrary sustain period Tsq, excepting the sustainperiod Tsp, and (where q is an arbitrary natural number greater than orequal to 1, and less than or equal to n, excepting p), the length of thesustain period Tsp is expressed as 2^(−(p−1))T×m (where T is a positiveconstant), and the length of the sustain period Tsq is expressed as2^(−(q−1))T.

According to the present invention, it may be characterized in that theplurality of EL elements each have a first electrode, a secondelectrode, and an EL layer formed between the first electrode and thesecond electrode and the EL layers contain a low molecular weightorganic material or an organic polymer material.

According to the present invention, there is provided an electro-opticaldevice comprising a plurality of EL elements and a plurality of pixelshaving the plurality of EL elements, wherein one frame period consistsof n subframe periods SF1, SF2, . . . , Sfn, the n subframe periods SF1,SF2, . . . , SFn have address periods Ta1, Ta2, . . . , Tan and sustainperiods Ts1, Ts2, . . . , Tsn, respectively, digital data signals in theaddress periods Ta1, Ta2, . . . , Tan are input to all of the pluralityof pixels, the plurality of EL elements are selected to emit light ornot to emit light during the sustain periods Ts1, Ts2, . . . , Tsn inaccordance with the digital data signals, the brightness of the lightemitted by the EL elements during at least one sustain period Tsp (wherep is a natural number greater than or equal to 1, and less than or equalto n), among the sustain periods Ts1, Ts2, . . . , Tsn, is 1/m (where mis a positive number) of the brightness of the light emitted by the ELelements during an arbitrary sustain period Tsq, excepting the sustainperiod Tsp, and (where q is an arbitrary natural number greater than orequal to 1, and less than or equal to n, excepting p), the length of thesustain period Tsp is expressed as 2^(−(p−1))T×m (where T is a positiveconstant), the length of the sustain period Tsq is expressed as2^(−(q−1))Tm, each the plurality of EL elements has a first electrode, asecond electrode, and an EL layer formed between the first electrode andthe second electrode and the brightness of the light emitted by the ELelements is controlled by an on EL driver voltage applied between thefirst electrode and the second electrode.

According to the present invention, it may be characterized in that theEL layers contain a low molecular weight organic material or an organicpolymer material.

According to the present invention, it may be characterized in that thelow molecular weight organic material is made from Alq3(tris-8-quinolinolate aluminum complex) or TPD (tri-phenylaminedielectric).

According to the present invention, it may be characterized in that theorganic polymer material is made from PPV (poly-paraphenylene vinylene),PVK (polyvinyl carbazole), or polycarbonate.

According to the present invention, one frame period may be equal to orless than 1/60 second.

According to the present invention, it may be characterized in that theelectro-optical device has a memory circuit for storing correction datain order to apply a correction to a display and the digital video signalcorrected by the memory circuit is input to a source signal side drivercircuit.

The present invention may be a computer, a video camera or a DVD playerusing the electro-optical device.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram of a pixel portion of an EL display of thepresent invention;

FIG. 2 is a timing chart of a digital time division gray scale displayof the present invention;

FIGS. 3A and 3B are graphs showing the transistor characteristics of anEL driver TFT;

FIGS. 4A and 4B are diagrams showing a circuit structure of an ELdisplay of the present invention;

FIG. 5 is a schematic diagram of a cross sectional structure of an ELdisplay of the present invention;

FIG. 6 is a timing chart of a digital time division gray scale displayof the present invention;

FIGS. 7A to 7E are diagrams showing a process of manufacturing an ELdisplay of the present invention;

FIGS. 8A to 8D are diagrams showing the process of manufacturing the ELdisplay of the present invention;

FIGS. 9A to 9D are diagrams showing the process of manufacturing the ELdisplay of the present invention;

FIGS. 10A to 10C are diagrams showing the process of manufacturing theEL display of the present invention;

FIG. 11 is a perspective view of an EL display of the present invention;

FIGS. 12A and 12B are a top view and a cross sectional diagram,respectively, of an EL display of the present invention;

FIGS. 13A and 13B are circuit diagrams of pixel portions of EL displaysof the present invention;

FIGS. 14A and 14B are circuit diagrams of pixel portions of EL displaysof the present invention;

FIGS. 15A and 15B are circuit diagrams of pixel portions of EL displaysof the present invention;

FIGS. 16A and 16B are circuit diagrams of pixel portions of EL displaysof the present invention;

FIGS. 17A to 17E are electronic equipment using an EL display of thepresent invention;

FIG. 18 is a circuit diagram of a pixel portion of an analog type ELdisplay;

FIG. 19 is a timing chart of an analog type EL display;

FIG. 20 is a graph showing the relationship between a pre-compensationvideo signal and a post-compensation video signal;

FIGS. 21A and 21B are compensation systems used in an EL display of thepresent invention; and

FIG. 22 is a diagram showing the relationship between a pre-compensationvideo signal and a post-compensation video signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The digital time division gray scale display of the present invention isexplained below using FIG. 1 and FIG. 2. A case of performing 2^(n) grayscale display in accordance with an n-bit digital data signal isexplained here.

The structure of a pixel portion 1500 of an EL display of the presentinvention is shown in FIG. 1. Gate signal lines (G1 to Gy) for inputtinggate signals are connected to gate electrodes of switching TFTs 1501 ofeach pixel. Further, one of a source region and a drain region of theswitching TFT 1501 of each pixel is connected to source signal lines(also referred to as data signal lines) (S1 to Sx) for inputting digitalsignals, and the other is connected to the gate electrode of an ELdriver TFT 1504 and to a capacitor 1508 of each pixel. Note that thisstructure in the embodiment mode has the capacitors 1508, but astructure not including the capacitors 1508 may also be used. There areno limitations upon the present invention by the presence of, or lackof, capacitors.

One of the source region and the drain region of the EL driver TFT 1504is connected to electric power source supply lines (V1 to Vx), and theother is connected to EL elements 1506. The electric potential of theelectric power source supply lines (V1 to Vx) is referred to as anelectric power source electric potential. Further, the power sourcesupply lines (V1 to Vx) are connected to the capacitors 1508 of eachpixel. Note that the digital data signal is a signal in which an analogor a digital video signal is transformed into a digital signal forperforming time division gray scale display, and that it contains imageinformation.

The EL elements 1506 are each composed of an anode, a cathode, and an ELlayer formed between the anode and the cathode. When the anodes areconnected to the source regions or the drain regions of the EL driverTFTs 1504, namely when the anodes are the pixel electrodes, then thecathodes become opposite electrodes. Conversely, for a case in which thecathodes are connected to the source regions or the drain regions of theEL driver TFTs 1504, namely when the cathodes are the pixel electrodes,then the anodes become the opposite electrodes. Furthermore, theelectric potential of the opposite electrodes is referred to as asteady-state electric potential within this specification. Note that anpower source which imparts the steady-state electric potential to theopposite electrodes is referred to as a steady-state power source.

The electric potential difference between the steady-state electricpotential of the opposite electrodes and the power source electricpotential of the pixel electrode is an EL driver voltage, and the ELdriver voltage is applied to the EL layers. The power source electricpotential is normally constant.

A timing chart during the digital drive of the EL display of the presentinvention is shown in FIG. 2. First, one frame period (F) is dividedinto n subframe periods (SF1 to SFn). Note that the period in which allof the pixels of the pixel portion display one image is referred to asone frame period (F).

In a normal EL display, 60 or more frame periods are formed during onesecond, and 60 or more images are displayed during one second. If thenumber of images displayed during one second becomes fewer than 60, thenproblems such as image flicker start to become visually conspicuous.

Note that a plurality of periods into which one frame period isadditionally divided are referred to as subframe periods. As the numberof gradations increases, the number of frame period partitionsincreases, and the driver circuit must be driven at a high frequency.

The subframe periods are divided into address periods (Ta) and sustainperiods (Ts). The address period is the time required for inputtingdigital data signals into all of the pixels during one subframe period,and the sustain period (also referred to as a turn on period) denotes aperiod in which the EL element is made to emit light.

The address periods (Ta) of SF1 to SFn become Ta1 to Tan, respectively.The sustain periods (Ts) of SF1 to SFn become Ts1 to Tsn, respectively.

First, in the address period, the opposite electrode of the EL elements1506 is held at the steady-state potential of the same height as thepower source electric potential. In the embodiment mode of the presentinvention, the steady-state potential in the address period of thedigital driver is referred to as an off steady-state potential. Notethat the height of the off steady-state potential may be the same as theheight of the power source electric potential within the range in whichthe EL elements 1506 do not emit light. Note that, the EL driver voltageat this time is referred to as the off EL driver voltage. Ideally, it isdesired that the off EL driver voltage is 0V, but the voltage may be onthe level on which the EL elements 1506 do not emit light.

Then, the gate signal is input to the gate signal line G1, and all theswitching TFTs 1501 connected to the gate signal line G1 are turned on.

When the switching TFTs 1501 connected to the gate signal line G1 areplaced into an On state, a digital data signal is input into all of thesource signal lines (S1 to Sx) at the same time. The digital data signalcontains “0” or “1” information, and one of the digital data signals “0”and “1” has Hi electric voltage, while the other has Lo voltage.

The digital data signal input into the source signal lines (S1 to Sx) isthen input to the gate electrodes of the EL driver TFTs 1504 through theswitching TFTs 1501, which are in the On state. Further, the digitaldata signal is also input to the capacitors 1508 of all of the pixelsconnected to the gate signal line G1, and an electric charge is stored.

Next, the gate signal is input to the gate signal line G2, and all ofthe switching TFTs 1501 connected to the gate signal line G2 are placedin the On state. With the switching TFTs 1501 connected to the gatesignal line G2 in the On state, the digital data signal is input to allof the source signal lines (S1 to Sx) at the same time. The digital datasignal input to the source signal lines (S1 to Sx) is input to the gateelectrodes of the EL driver TFTs 1504 through the switching TFTs 1501.Further, the digital data signal is also input to the capacitors 1508 ofall of the pixels connected to the gate signal line G2, and stored.

By repeating the above operations through the gate signal line Gy, thedigital data signal is input to all of the pixels. The period up untilthe digital data signal is input to all of the pixels is the addressperiod.

The sustain period commences at the same time as the address period iscomplete. When the sustain period begins, the electric potential of theopposite electrodes changes from an off steady-state electric potentialto an on steady-state electric potential. The steady-state electricpotential in the sustain period of the digital drive is referred to asthe on steady-state electric potential in the embodiment mode of thepresent invention. The on steady-state electric potential may have anelectric potential difference with the power source electric potentialto the extent that the EL elements emit light. Note that this electricpotential difference is referred to as an on EL driver voltage. Notealso that the off steady-state electric potential and the onsteady-state electric potential are generically referred to assteady-state electric potentials. Further, the on EL driver voltage andan off EL driver voltage are generically referred to as EL drivervoltages.

All of the switching TFTs 1501 are set into the Off state in the sustainperiod. The digital data signal stored in the capacitors 1508 is theninput to the gate electrodes of the EL driver TFTs 1504.

When the digital data signal contains “0” information, then the ELdriver TFTs 1504 are set into the Off state in the embodiment mode ofthe present invention. The pixel electrodes of the EL elements 1506 aretherefore maintained at the off steady-state electric potential. As aresult, the EL elements 1506 contained in pixels to which the digitaldata signal containing “0” information is applied do not emit light.

On the other hand, for cases having “1” information, the EL driver TFTs1504 turn on in the embodiment mode of the present invention. Theelectric power source electric potential is therefore imparted to thepixel electrodes of the EL elements 1506. As a result, EL elements 1506contained in pixels to which the digital data signal having “1”information emit light.

An address period again begins at the completion of the sustain period,and when the data signal is input to all of the pixels, a sustain periodbegins. Any of the periods Ts1 to Ts(n−1) become the sustain period atthis point. The Ts(n−1) period turns on predetermined pixels here.

Similar operations are repeated subsequently for the remaining n−2subframe periods, Ts(n−2), Ts(n−3), . . . , Ts1 are set, in order, assustain periods, and predetermined pixels are turned on in therespective subframes.

One frame period is complete after the n subframe periods are completed.

Note that, in the present invention, among the n sustain periods Ts1, .. . Tsn, the brightness of light emitted by the EL elements during atleast one sustain period is set to be always lower than the brightnessof light emitted by the EL elements in other sustain periods.

If a sustain period in which the brightness of light emitted is 1/m ofthat emitted in other sustain periods is taken as Tsp (where p is anarbitrary number greater than or equal to 1 and less than or equal ton), then among the n sustain periods Ts1, . . . Tsn, the length of thesustain periods other than the sustain period Tsp is expressed as2^(−(n−1)) T, where T is a positive constant. Further, the length of thesustain period Tsp is expressed as 2^(−(p−1)) T×m. Note that m is apositive number greater than 1. Therefore, even if the brightness oflight emitted by the EL elements during the sustain period Tsp is 1/mthat emitted during the other sustain periods, the length of the sustainperiod Tsp is set to be 2^(−(p−1)) T×m, and the predetermined gray scaledisplay can therefore be obtained.

Consequently, whichever of the n sustain periods Ts1, . . . , Tsn istaken as the sustain period Tsp, and however many sustain periods Tspare formed, if the amount of light emitted by the EL elements duringeach of the sustain periods Ts1, . . . Tsn is taken as Lm1, . . . , Lmn,then Lm1:Lm2:Lm3: . . . :Lm(n−1):Lmn=2⁰:2⁻¹:2⁻²: . . .:2^(−(n−2)):2^(−(n−1)). Note that SF1 to Sfn may appear in any order,and therefore the order of appearance of the sustain periods Ts1, . . ., Tsn is also arbitrary. By combining the sustain periods, the desiredgray scale display, among the 2^(n) gradations, can be performed.

The gradation of each pixel is determined by which subframe periods areselected for light emission during one frame period. For example, ifn=8, and the brightness of pixels having light emitted during all of thesustain periods is taken as 100%, then for a case of the pixels emittinglight in Ts1 and Ts2, the brightness is expressed as 75%, and when Ts3,TS5, and Ts8 are selected, the brightness can be expressed as 16%.

With the present invention, even if the Id-Vg characteristics of theTFTs vary slightly, dispersion in the amount of current output whenequal gate voltages are applied can be suppressed by the abovestructure. It therefore becomes possible to avoid the situation in whichthe amount of light emitted by EL elements of adjacent pixels differsgreatly due to variations in the Id-Vg characteristics, even with thesame voltage signal input.

Further, the amount of time in which light is emitted by the EL elementscan be set to 2^(−(p−1)) T×m (where T is a positive constant) in thesustain period Tsp, in which the brightness of light emitted by the ELelements is 1/m of the brightness emitted during other sustain periods.By using the above structure, and by increasing the number of imagegradations, the number of bits n becomes larger, and even if the lengthof the sustain period expressed by 2^(−(n−1)) T becomes shorter, thebrightness of light emitted by the EL elements is regulated to be 1/mthat emitted in other sustain periods, the length of the sustain periodis set to 2^(−(p−1)) T×m, and it becomes possible to extend it.

Note that with the above structure of the present invention, theelectric power source electric potential is always maintained constant,the electric potential of the opposite electrodes is changed by theaddress period and the sustain period, and the off EL driver voltage orthe on EL driver voltage is applied to the EL layers. However, thepresent invention is not limited to this structure. Alternatively, theelectric potential of the opposite electrodes may be always maintainedconstant, and by changing the power source electric potential by theaddress period and the sustain period, the off EL driver voltage or theon EL driver voltage may be applied to the EL layers. In this case,regulation of the brightness of the EL elements is performed bycontrolling the power source electric potential.

Further, with the above structure of the present invention, the off ELdriver voltage is taken as zero and the EL elements are not made to emitlight, but the off EL driver voltage may also be set to the same voltageas the on EL driver voltage and light may also be emitted during theaddress period. In this case, the power source electric potential andthe steady-state electric potential are always maintained at fixedvalues. However, in this case the subframe period becomes a period ofemitting light, and therefore the length of the subframe periods is setto SF1, SF2, . . . , Sfn=2⁰T, 2⁻¹T, . . . , 2^(−(n−1))T, and the lengthof the subframe period having a brightness of 1/m is set to2^(−(n−1))T×m. An image having a high brightness compared to that of adriver method in which light is not emitted during the address periodcan be obtained by the above structure.

Furthermore, in the embodiment mode of the present invention, there isexplained a case where the device is driven with the non-interlacescanning, but the device of the invention can also be driven with theinterlace scanning.

EMBODIMENTS

Embodiments of the present invention will be explained hereinbelow.

Embodiment 1

The embodiments of the present invention are explained in the following.

The structure of the present invention is explained using an example ofan EL display which performs time partitioning gradation display ofdigital driving method. An example of a circuit structure of the presentinvention is shown in FIG. 4.

The EL display device of FIG. 4A has a pixel portion 101, and a sourcesignal side driver circuit 102 and a gate signal side driver circuit 103arranged in the periphery of the pixel portion, which are all composedof TFTs formed on a substrate. Note that although the EL display devicehas one source signal side driver circuit and one gate signal sidedriver circuit in the embodiment 1 of the present invention, two sourcesignal side driver circuits may also be used in the present invention.Furthermore, two gate signal side driver circuits may also be used.

The source signal side driver circuit 102 fundamentally contains a shiftregister 102 a, a latch (A) 102 b and a latch (B) 102 c. Further, clockpulses (CK) and start pulses (SP) are input to the shift register 102 a,digital data signals are input to the latch (A) 102 b, and latch signalsare input to the latch (B) 102 c.

Further, not shown in a figure, gate signal side driver circuit 103 haveshift register and buffer. A multiplexer can be provided in the outputside of the buffer.

The digital data signals input to the pixel portion are formed by a timepartitioning gradation data signal generator circuit 114. This circuit,along with converting a video signal which is an analog signal or adigital signal (a signal containing image information) into a digitaldata signal for performing time partitioning gradation, is a circuitwhich generates timing pulses or the like which are necessary forperforming time partitioning gradation display.

Typically, a means of partitioning one frame period into a plurality ofsub-frame periods corresponding to n-bit gradations (where n is a realnumber equal to or greater than 2), a means of selecting an addressperiod and a sustain period in the plurality of sub-frame periods, and ameans of setting the sustain periods.

The time partitioning gradation data signal generator circuit 114 mayalso be formed external to the EL display device of the presentinvention. In this case, it becomes a structure that the digital datasignals formed there are input to the EL display device of the presentinvention. An electronic device (EL display device) with the EL displaydevice of the present invention as a display thus contains the ELdisplay device of the present invention and the time partitioninggradation data signal generator circuit as separate components.

Further, the EL display device of the present invention may also beimplemented with a form such as using an IC chip for the timepartitioning gradation data signal generator circuit 114. In this case,it becomes a structure that the digital data signals formed by the ICchip are input to the EL display device of the present invention. Anelectronic device with the EL display device of the present invention asa display thus contains the EL display device of the present inventionin which the IC chip containing the time partitioning gradation datasignal generator circuit is implemented.

Further, the time partitioning gradation data signal generator circuit114 may also be formed of TFTs on the same substrate as the pixelportion 101, the source signal side driver circuit 102, and the gatesignal side driver circuit 103. In this case, all processing can beperformed on the substrate provided that the video signal containingimage information is input to the EL display device. It can be of courseto form the time partitioning gradation data signal generator circuitwith TFTs having polysilicon films used in the present invention asactive layers. Furthermore, in this case an electronic device having theEL display device of the present invention as a display has the timepartitioning gradation data signal generator circuit incorporated withinthe EL display device itself, and it is possible to make the electronicdevice small.

A plurality of pixels 104 are arranged in a matrix state in the pixelportion 101. An expanded diagram of the pixels 104 is shown in FIG. 4B.Reference numeral 105 denotes a switching TFT in FIG. 4B. A gateelectrode of the switching TFT 105 is connected to a gate signal line106 which inputs a gate signal. One of a drain region and a sourceregion of the switching TFT 105 is connected to a source signal line 107for inputting a digital data signal, and the other is each connected toa gate electrode of an EL driver TFT 108 and capacitor 113 which ispossessed by each pixel.

Further, a source region of the EL driver TFT 108 is connected to anelectric power supply line 111 and a drain region is connected to an ELelement 110. An electric power supply line 111 is connected to thecapacitor 113. The capacitor 113 is provided to keep the gate voltage ofan EL driver TFT 108 when the switching TFT 105 is non-selected state(Off state).

The EL element 110 is composed of an anode and a cathode, and an ELlayer which is provided between an anode and a cathode. An anode as anopposing electrode is maintained at a fixed electric potential when thecathode is connected to a source region or the drain region of the ELdriver TFT 108, in other words, the cathode is a pixel electrode. On thecontrary, an anode as an opposing electrode is maintained a fixedelectric potential when the cathode is connected to a source region orthe drain region of the EL driver TFT 108, in other words, the cathodeis a pixel electrode.

The electric power supply line 111 is maintain the power supplypotential.

Note that a resistive body may also be formed between the drain regionor the source region of the EL driver TFT 108 and the EL element 110. Itbecomes possible to control the amount of current supplied to the ELelement from the EL driver TFT, and to prevent any influence resultingin dispersion of the characteristics of the EL driver TFT with formingthe resistive body. The resistive body may be an element displaying aresistance sufficiently larger than the on resistance of the EL driverTFT 108, and therefore there are no limitations in its structure. Notethat the on resistance is the value of the drain voltage of the TFTdivided by the drain current flowing at that the time. The resistancevalue of the resistive body may be selected in a range of 1 kΩ to 50 MΩ(preferably between 10 kΩ and 10 MΩ, more preferably between 50 kΩ and 1MΩ). Use of a semiconductor layer having high resistance value as theresistive body is preferably because it is easy to form.

Next, reference is made to FIG. 5 schematically showing the sectionalstructure of the EL display device of the present invention.

In FIG. 5 reference numeral 11 is a substrate, and 12 is an insulatingfilm that is a base (hereinafter, this film is designated as base film).For the substrate 11, use can be made of a light transmissiblesubstrate, representatively, a glass substrate, a quartz substrate, aglass ceramic substrate, or a crystallization glass substrate. However,it must be resistible to the highest processing temperature in amanufacturing process.

The base film 12 is effective especially in using a substrate that has amovable ion or a substrate that has conductivity, but it is notnecessarily disposed on the quartz substrate. An insulating film thatcontains silicon can be used as the base film 12. It should be notedthat, in this specification, “insulating film that contains silicon”signifies an insulating film in which oxygen or nitrogen is added tosilicon at a predetermined ratio (SiOxNy: x and y are arbitraryintegers), such as a silicon oxide film, a silicon nitride film or asilicon nitride oxide film.

Reference numeral 201 is a switching TFT, and 202 is a EL driver TFT. Aswitching TFT is formed by an n-channel type TFT. An EL driver is formedby a p-channel TFT. When the EL light luminescence directed to the undersurface (the surface which is not provided a TFT or an EL layer) of thesubstrate, the above structure is preferable. However, in the presentinvention, there is no need to limit the above structure. It is alsopossible to use both the n-channel type TFT and p-channel type TFT for aswitching TFT or an EL driver TFT.

The switching TFT 201 is made up of an active layer that includes asource region 13, a drain region 14, LDD regions 15 a-15 d, an isolationregion 16, and channel formation regions 17 a, 17 b, a gate insulatingfilm 18, gate electrodes 19 a, 19 b, a 1st interlayer insulating film20, a source wiring line (a part of a source signal line) 21, and adrain wiring line 22. The gate insulating film 18 or the 1st interlayerinsulating film 20 can be common to all TFTs on the substrate, or can bevaried according to circuits or elements.

In the switching TFT 201 shown in FIG. 5, the gate electrodes 19 a, 19 bare connected electrically, in other words, a so-called double gatestructure is established. Not only the double gate structure but also aso-called multi gate structure, such as a triple gate structure, can beestablished, of course. The multi gate structure signifies a structureincluding an active layer that has two channel formation regions or moreconnected in series.

The multi gate structure is very effective to decrease an OFF-statecurrent, and if the OFF-state current of the switching TFT is decreasedsufficiently, the capacity necessary for the capacitor which isconnected to a gate electrode of an EL driver TFT 202 can be reduced.That is, since the possession area of the capacitor can be reduced, themulti gate structure is also effective to widen the effectiveluminescence area of the EL element.

In the switching TFT 201, the LDD regions 15 a-15 d are disposed not tooverlap with the gate electrodes 19 a and 19 b, with the gate insulatingfilm 18 therebetween. The thus built structure is very effective todecrease the OFF-state current. The length (width) of the LDD regions 15a-15 d is 0.5-3.5 mm, representatively, 2.0-2.5 mm.

It is more desirable to form an offset region (i.e., region formed witha semiconductor layer whose composition is the same as the channelformation region, and in which a gate voltage is not applied) betweenthe channel formation region and the LDD region, in order to decreasethe OFF-state current. In the multi gate structure that has two gateelectrodes or more, the isolation region 16 (i.e., region whoseconcentration is the same and to which the same impurity element isadded as the source region or the drain region) formed between thechannel formation regions is effective to decrease the OFF-statecurrent.

The EL driver TFT 202 is made up of an active layer that includes asource region 26, a drain region 27, and a channel formation region 29,a gate insulating film 18, a gate electrode 30, the 1st interlayerinsulating film 20, a source wiring line 31, and a drain wiring line 32.The EL driver TFT 202 is a p-channel type TFT.

The drain region 14 of the switching TFT is connected to the gateelectrode 30 of the EL driver TFT 202. In more detail, but not shown infigures, the gate electrode 30 of the EL driver TFT 202 is connectedelectrically to the drain region 14 of the switching TFT 201 through thedrain wiring line 22 (also called connection wiring line). While thegate electrode 30 is a single gate structure here, the multi gatestructure is also applied. The source wiring line 31 of the EL drivingTFT 202 is connected to the current-feed line (not shown).

The EL driver TFT 202 is an element to control the amount of currentsupplied to the EL element, and a comparatively large amount of currentcan flow therethrough. Therefore, preferably, the channel-width (W) isdesigned to be greater than the channel-width of the switching TFT.Additionally, preferably, the channel-length (L) is designed to be longso that an excessive current does not flow through the currentcontrolling TFT 202. A desirable value is 0.5-2 mA (1-1.5 mA preferably)per pixel.

From the viewpoint of restraining the deterioration of the EL driverTFT, it is also effective to thicken the film thickness of the activelayer (specifically, the channel formation region) of the EL driver TFT202 (50-100 nm preferably, and 60-80 nm further preferably). On theother hand, from the viewpoint of decreasing the OFF-state current inthe switching TFT 201, it is also effective to thin the film thicknessof the active layer (specifically, the channel formation region)(20-50nm preferably, and 25-40 nm further preferably).

The structure of the TFT formed in the pixel was described above. Inthis formation, a driving circuit is also formed at the same time. ACMOS circuit that is a base unit to form the driving circuit is shown inFIG. 5.

In FIG. 5, a TFT that has a structure to decrease the hot carrierinjection without reducing the operation speed to the utmost is used asthe n-channel type TFT 204 of the CMOS circuit. The driving circuitdescribed herein is the source signal side driving circuit and the gatesignal side driving circuit. It is also possible to form other logiccircuits (level shifter, A/D converter, signal division circuit, etc.),of course.

The active layer of the n-channel type TFT 204 of the CMOS circuitincludes a source region 35, a drain region 36, an LDD region 37, and achannel formation region 38. The LDD region 37 overlaps with the gateelectrode 39, with the gate insulating film 18 therebetween.

The reason for forming the LDD region 37 only on the drain region 36side is not to reduce the operation speed. There is no need to worryabout the OFF-state current value in the n-channel type TFT 204.Instead, the operation speed should be rated above it. Therefore,preferably, the LDD region 37 is completely laid on the gate electrode,thus reducing a resistance component as much as possible. That is, aso-called offset should be omitted.

In the p-channel type TFT 205 of the CMOS circuit, there is no need toprovide the LDD region especially because the deterioration caused bythe hot carrier injection is quite negligible. Therefore, the activelayer includes a source region 40, a drain region 41, and a channelformation region 42. The gate insulating film 18 and the gate electrode43 are disposed thereon. It is also possible to dispose the LDD regionas well as the n-channel type TFT 204 in order to take countermeasuresagainst the hot carrier, of course.

The n-channel type TFT 204 and the p-channel type TFT 205 are coveredwith the first interlayer insulating film 20, and the source wiringlines (a part of the source signal lines) 44, 45 are formed. Then-channel type TFT 204 and the drain region of the p-channel type TFT205 are connected electrically by the drain wiring line 46.

Reference numeral 47 is a first passivation film. The film thicknessthereof is 10 nm-1 mm (200-500 nm preferably). An insulating filmincluding silicon (especially, a silicon nitride oxide film or a siliconnitride film is desirable) can be used as its material. The passivationfilm 47 serves to protect a formed TFT from alkali metal and water. TheEL layer finally disposed above the TFT (especially the EL driver TFT)includes alkali metal such as sodium. In other words, the firstpassivation film 47 serves also as a protective layer by which thealkali metal (movable ions) is not allowed to enter the TFT side.

Reference numeral 48 is a second interlayer insulating film, and servesas a flattening film to flatten level differences formed by the TFT.Preferably, an organic resin film, such as polyimide, polyamide, acrylicresin, or BCB (benzocyclobutene) is used as the second interlayerinsulating film 48. These films have an advantage in that a good smoothplane can be easily formed, and the dielectric constant is low. It ispreferable to entirely absorb the level difference caused by the TFT bymeans of the second interlayer insulating film 48 because the EL layeris very sensitive to ruggedness. Additionally, it is preferable to forma low-dielectric constant material thick, in order to decrease theparasitic capacitance formed between the gate signal line or the datasignal line and the cathode of the EL element. Therefore, preferably,the film thickness thereof is 0.5-5 mm (1.5-2.5 mm preferably).

Reference numeral 49 is a pixel electrode (anode of the EL element) thatis made of a transparent conductive film. After a contact hole (opening)is made in the second interlayer insulating film 48 and the firstpassivation film 47, the electrode is connected to the drain wiring line32 of the EL driver TFT 202 through the opening. When the pixelelectrode 49 and the drain region 27 are arranged not to be connecteddirectly, as in FIG. 5, the alkali metal of the EL layer can beprevented from entering the active layer via the pixel electrode.

A third interlayer insulating film 50 whose thickness is 0.3-1 mm isdisposed on the pixel electrode 49. The film 50 is made of a siliconoxide film, a silicon nitride oxide film, or an organic resin film. Thethird interlayer insulating film 50 is provided with an opening on thepixel electrode 49 by etching, and the edge of the opening is etched tohave a taper shape. Preferably, the angle of the taper is 10-60° (30-50°preferably).

An EL layer 51 is formed on the third interlayer insulating film 50. TheEL layer 51 is used in the form of a single-layer structure or a layeredstructure. The layered structure is superior in luminous efficiency.Generally, a positive hole injection layer/a positive hole transportinglayer/a luminescent layer/an electronic transporting layer are formed onthe pixel electrode in this order. Instead, a structure may be usedwhich has the order of positive hole transporting layer/luminescentlayer/electronic transporting layer or the order of positive holeinjection layer/positive hole transporting layer/luminescentlayer/electronic transporting layer/electronic injection layer. In thepresent invention, any one of the known structures can be used, andfluorescent coloring matter, etc., can be doped to the EL layer.

For example, materials indicated in the following U.S. patents orpublications can be used as the organic EL material; U.S. Pat. Nos.4,356,429: 4,539,507: 4,720,432: 4,769,292: 4,885,211: 4,950,950:5,059,861: 5,047,687: 5,073,446: 5,059,862: 5,061,617: 5,151,629:5,294,869: 5,294,870, and Japanese Laid-Open Patent Publication Nos.189525 of 1998: 241048 of 1996: 78159 of 1996.

The EL display device mainly has four color display methods; method offorming three kinds of EL elements that correspond to R(red), G(green),and B(blue), respectively; method of combining an EL element of whiteluminescence and a color filter; method of combining an EL element ofblue or blue-green luminescence and a fluorescent body (fluorescentcolor conversion layer: CCM); and method of stacking the EL elementsthat correspond to RGB while using a transparent electrode for a cathode(opposite electrode).

The structure of FIG. 5 is an example in which the method of formingthree kinds of EL elements that correspond to RGB is used. Only onepixel is shown in FIG. 5. In fact, pixels, each having the samestructure, are formed to correspond to each color of red, green, andblue, and thereby color display can be performed.

The present invention can be performed regardless of the luminescencemethod, and can use all the four methods. However, since the speed ofresponse of the fluorescent body is slower than that of the EL, and theproblem of afterglow occurs, the method in which the fluorescent body isnot used is preferable. Additionally, it can be said that a color filterthat causes the fall of luminescence brightness should not be used ifpossible.

A cathode 52 of the EL element is disposed on the EL layer 51. Amaterial that includes magnesium (Mg), lithium (Li) or calcium (Ca) thatis small in work function is used as the cathode 52. Preferably, use ismade of an electrode made of MgAg (material in which Mg and Ag are mixedin the ratio of Mg:Ag=10:1). Instead, a MgAgAl electrode, a LiAlelectrode, or LiFAl electrode can be used.

In this specification, the luminescence element formed by the pixelelectrode (anode), the EL layer, and the cathode is called an EL element206.

It is necessary to form a layered body comprised of the EL layer 51 andthe cathode 52 by each pixel individually. However, the EL layer 51 isquite weak to water, and a normal photolithography technique cannot beused. Therefore, it is preferable to use a physical mask material, suchas metal mask, and selectively form it according to a vapor phasemethod, such as a vacuum deposition method, a sputtering method, or aplasma CVD method.

It is also possible to use an ink jet method, a screen printing method,spin court method, and the like, as the method of selectively formingthe EL layer. However, these methods cannot continuously form thecathode in the current state of the art, and it can be said that themethod described above, not the ink jet method, etc., is desirable.

Reference numeral 53 is a protective electrode. This is to protect thecathode 52 from outside water, etc., and, at the same time, connect thecathode 52 of each pixel. For the protective electrode 53, it ispreferable to use a low-resistance material including aluminum (Al),copper (Cu), or silver (Ag). A cooling effect to lower the heat of theEL layer can be expected from the protective electrode 53. It is alsoeffective to continue to the protective electrode 53 without airexposure after the above mentioned cathode 52 are formed.

Reference numeral 54 is a second passivation film, and, preferably, thefilm thickness thereof is 10 nm-1 mm (200-500 nm preferably). A mainpurpose to dispose the second passivation film 54 is to protect the ELlayer 51 from water. It is also effective to give it a cooling effect.However, the EL layer is weak to heat as mentioned above, and filmformation should be performed at a low temperature (ranging from a roomtemperature to 120° C. preferably). Therefore, it can be said that adesirable film formation method is the plasma CVD method, sputteringmethod, vacuum deposition method, ion plating method, or solutionapplication method (spin court method).

Needless to say, all the TFTs shown in FIG. 5 have the polysilicon filmsused in the present invention as active layers.

Therefore, the present invention is not limited to the structure of theEL display device of FIG. 5, which is just one of the preferredembodiments.

Embodiment 2

The digital time division gray scale display method of the presentinvention is explained below using FIG. 6. A case of performing 2⁴ grayscale display in accordance with a 4 bit digital data signal isexplained here.

The structure of the pixel portion of the EL display of the presentinvention is the same as that shown in FIG. 1. Gate signal lines (G1 toGy) for inputting a gate signal are connected to gate electrodes ofswitching TFTs in each pixel. Further, one of a source region and adrain region of the switching TFT of each pixel is connected to sourcesignal lines (also referred to as data signal lines) (S1 to Sx) forinputting digital signals and the other is connected to the gateelectrode of an EL driver TFT and to a capacitor, of each pixel. Notethat this structure in Embodiment 2 has capacitors, but a structure notincluding capacitors may also be used. There are no limitations upon thepresent invention by the presence of, or lack of, capacitors.

One of the source region and the drain region of the EL driver TFT isconnected to electric power source supply lines (V1 to Vx), and theother is connected to EL elements. The electric potential of the powersource supply lines (V1 to Vx) is referred to as an electric powersource electric potential. Further, the electric power source supplylines (V1 to Vx) are connected to the capacitors of each pixel.

The EL elements are each composed of an anode, a cathode, and an ELlayer formed between the anode and the cathode. When the anodes areconnected to the source regions or the drain regions of the EL driverTFTs, namely when the anodes are the pixel electrodes, then the cathodesbecome opposite electrodes. Conversely, for a case in which the cathodesare connected to the source regions or the drain regions of the ELdriver TFTs, namely when the cathodes are the pixel electrodes, then theanodes become the opposite electrodes. Furthermore, the electricpotential of the opposite electrodes is referred to as a steady-stateelectric potential within this specification.

The electric potential difference between the steady-state electricpotential of the opposite electrodes and the power source electricpotential of the pixel electrodes is an EL driver voltage, and the ELdriver voltage is applied to the EL layers.

A timing chart for the EL display digital drive of the present inventionis shown in FIG. 6. First, one frame period (F) is divided into foursubframe periods (SF1 to SF4). Note that a period in which all of thepixels of the pixel portion display one image is referred to as oneframe period (F).

The subframe periods are divided into address periods (Ta) and sustainperiods (Ts). The address period is the time required for inputtingdigital data signals into all of the pixels during one subframe period,and the sustain period (also referred to as a turn on period) denotes aperiod in which the EL element is made to emit light.

The address periods (Ta) of SF1 to SF4 become Ta1 to Ta4, respectively.The sustain periods (Ts) of SF1 to SF4 become Ts1 to Ts4, respectively.

First, in the address period, the opposite electrode is held at thesteady-state potential of the same height as the power source electricpotential. In the present embodiment of the present invention, thesteady-state potential in the address period of the digital driver isreferred to as an off steady-state potential. Note that, the height ofthe off steady-state potential may be the same as the height of thepower source electric potential within the range in which the ELelements do not emit light. Note that, the EL driver voltage at thistime is referred to as the off EL driver voltage. Ideally, it is desiredthat the off EL driver voltage is 0V, but the voltage may be on thelevel on which the EL elements do not emit light.

Then, the gate signal is input to the gate signal line G1, and all theswitching TFTs connected to the gate signal line G1 are turned on.

When the switching TFTs connected to the gate signal line G1 are placedinto an On state, a digital data signal is input into all of the sourcesignal lines (S1 to Sx) at the same time. The digital data signalcontains “0” or “1” information, and one of the digital data signals “0”and “1” has Hi electric voltage, while the other has Lo voltage.

The digital data signal input into the source signal lines (S1 to Sx) isthen input to the gate electrodes of the EL driver TFTs through theswitching TFTs, which are in the On state. Further, the digital datasignal is also input to the capacitors of all of the pixels connected tothe gate signal line G1, and an electric charge is stored.

Next, the gate signal is input to the gate signal line G2, and all ofthe switching TFTs connected to the gate signal line G2 are placed inthe On state. With the switching TFTs connected to the gate signal lineG2 in the On state, the digital data signal is input to all of thesource signal lines (S1 to Sx) at the same time. The digital data signalinput to the source signal lines (S1 to Sx) is input to the gateelectrodes of the EL driver TFTs through the switching TFTs. Further,the digital data signal is also input to the capacitors of all of thepixels connected to the gate signal line G2, and stored.

By repeating the above operations through the gate signal line Gy, thedigital data signal is input to all of the pixels. The period up untilthe digital data signal is input to all of the pixels is the addressperiod.

The sustain period commences at the same time as the address period iscomplete. When the sustain period begins, the electric potential of theopposite electrodes changes from an off steady-state electric potentialto an on steady-state electric potential. The steady-state electricpotential in the sustain period of the digital drive is referred to asthe on steady-state electric potential in the embodiments of the presentinvention. The on steady-state electric potential may have an electricpotential difference with the power source electric potential to theextent that the EL elements emit light. Note that this electricpotential difference is referred to as an on EL driver voltage. Notealso that the off steady-state electric potential and the onsteady-state electric potential are generically referred to assteady-state electric potentials. Further, the on EL driver voltage andan off EL driver voltage are generically referred to as EL drivervoltages.

All of the switching TFTs are set into the Off state in the sustainperiod. The digital data signal stored in the capacitors is then inputto the gate electrodes of the EL driver TFTs.

When the digital data signal contains “0” information, then the ELdriver TFTs are set into the Off state in the embodiments of the presentinvention. The pixel electrodes of the EL elements are thereforemaintained at the off steady-state electric potential. As a result, theEL elements contained in pixels to which the digital data signalcontaining “0” information is applied do not emit light.

On the other hand, for cases having “1” information, the EL driver TFTsturn on in the embodiments of the present invention. The electric powersource electric potential is therefore imparted to the pixel electrodesof the EL elements. As a result, EL elements contained in pixels towhich the digital data signal having “1” information emit light.

An address period again begins at the completion of the sustain period,and when the data signal is input to all of the pixels, a sustain periodbegins. Any of the periods Ts1 to TS become the sustain period at thispoint. The Ts3 period turns on predetermined pixels here.

Similar operations are repeated subsequently for the remaining 2subframe periods, Ts2 and Ts1 are set, in order, as sustain periods, andpredetermined pixels are turned on in the respective subframes.

One frame period is complete after the 4 subframe periods are completed.

Note that, in the present invention, among the four sustain periods Ts1,. . . Ts4, the brightness of light emitted by the EL elements during atleast one sustain period is set to be always lower than the brightnessof light emitted by the EL elements in other sustain periods. InEmbodiment 2, the brightness of light emitted by the EL elements duringthe sustain periods Ts3 and Ts4 is one-half the brightness of lightemitted by the EL elements during the sustain periods Ts1 and Ts2. Inother words, the on EL driver voltage during the sustain periods Ts3 andTs4 is one-half the EL driver voltage during the other sustain periodsTs1 and Ts2.

The length of the sustain periods Ts1 and Ts2, the sustain periods otherthan the sustain periods Ts3 and Ts4 having an emitted light brightnessof one-half that of Ts1 and Ts2, is expressed as T and 2⁻¹ T,respectively, where T is a positive constant. Further, the length of thesustain periods Ts3 and Ts4 is expressed as 2⁻² T×2 and 2⁻³ T×2,respectively. Namely, the ratio of lengths of the sustain periods Ts1 toTs4 is 1:2⁻¹:2⁻¹:2⁻². Therefore, even if the brightness of light emittedby the EL elements during the sustain periods Ts3 and Ts4 is one-halfthat of the light emitted during the other sustain periods Ts1 and Ts2,the ratio of lengths of the sustain periods Ts3 and Ts4 to all of thesustain periods is twice that of a case in which the brightness of lightemitted is not set to one-half. Therefore, even though the brightness oflight emitted by the EL elements in the sustain periods Ts3 and Ts4 isone half that of the other sustain periods, the length ratio of thesustain periods Ts3 and Ts4 is set to twice as long, and the desiredgray scale display can be obtained.

Consequently, even though the brightness of light emitted by the ELelements in the sustain periods Ts3 and Ts4 is one half that of theother sustain periods, whichever of the sustain periods Ts1, . . . , Ts4is given a reduced brightness, and however much the brightness isreduced, and however many low brightness sustain periods are formed, ifthe amount of light emitted by the EL elements during each of thesustain periods Ts1, . . . Ts4 is taken as Lm1, . . . , Lm4, then Lm1: .. . :Lm4=2⁰:2⁻¹:2⁻²:2⁻³. Note that SF1 to Sf4 may appear in any order,and therefore the order of appearance of the sustain periods Ts1, . . ., Ts4 is also arbitrary. By combining the sustain periods, the desiredgray scale display, from among the 2⁴ gradations, can be performed.

The gradation of each pixel is determined by which subframe periods areselected for light emission during one frame period. For example, ifn=4, and the brightness of pixels having light emitted during all of thesustain periods is taken as 100%, then for a case of the pixels emittinglight in Ts1 and Ts2, the brightness is expressed as 80%, and when Ts2,Ts3, and Ts4 are selected, the brightness can be expressed as 47%.

With the present invention, even if the Id-Vg characteristics of theTFTs vary slightly, dispersion in the amount of current output whenequal gate voltages are applied can be suppressed by the abovestructure. It therefore becomes possible to avoid the situation in whichthe amount of light emitted by EL elements of adjacent pixels differsgreatly due to variations in the Id-Vg characteristics, even with thesame video signal input.

Further, in a sustain period Tsp in which the brightness of lightemitted is 1/m of that emitted in other sustain periods Ts1 to Tsn, ifthe length of the other sustain periods Ts1 to Tsn is taken as2^(−(n−1)) T, where T is a positive constant, then the amount of time inwhich light is emitted by the EL elements can be taken as 2^(−(p−1))T×m. In accordance with the above structure, by regulating thebrightness of light emitted by the EL elements during the sustain periodTsp to be 1/m that emitted during the other sustain periods, the lengthratio of the sustain period Tsp to all of the sustain periods can beextended by a multiple of m compared is to a case in which thebrightness of light emitted during the sustain period Tsp is not set to1/m. Therefore, in accordance with increasing the number of imagegradations, even if the number of bits n becomes larger and the lengthof the sustain periods becomes shorter, the length of the sustainperiods can be extended by lowering the brightness of the light emittedby the EL elements.

Further, an example of driving by non-interlaced scanning is explainedin Embodiment 2, but it is also possible to drive by interlacing.

Note that it is possible to freely combine Embodiment 2 with Embodiment1.

Embodiment 3

In this embodiment, a description is here given of a method ofsimultaneously manufacturing TFTs of a pixel portion and a drivingcircuit portion around the pixel portion. Concerning the drivingcircuit, a CMOS circuit that is a base unit is shown in the figure, fora brief description.

First, a substrate 501 in which a base film (not shown) is disposed onthe surface thereof is prepared as shown in FIG. 7(A). In thisembodiment, a silicon nitride oxide film whose thickness is 200 nm andanother silicon nitride oxide film whose thickness is 100 nm arelaminated and are used as a base film on a crystallization glass. Atthis time, preferably, the concentration of nitrogen of the filmcontacting the crystallization glass substrate is kept to 10-25 wt %. Itis possible to form an element directly on the quartz substrate withoutany base film, of course.

Thereafter, an amorphous silicon film 502 whose thickness is 45 nm isformed on the substrate 501 by a well-known film formation method. Thereis no need to limit it to the amorphous silicon film. Instead, asemiconductor film (including a microcrystal semiconductor film) thathas an amorphous structure can be used in this embodiment. A compoundsemiconductor film that has an amorphous structure, such as an amorphoussilicon germanium film, also can be used herein.

Concerning the steps from here to FIG. 7(C), it is possible tocompletely cite Japanese Laid-open Patent Publication No. 247735 of 1998filed by the present applicant. This publication discloses a techniqueconcerning a method of crystallizing a semiconductor film, which uses anelement, such as Ni, as a catalyst.

First, a protective film 504 that has openings 503 a and 503 b isformed. A silicon oxide film 150 nm thick is used in this embodiment. Alayer 505 (Ni containing layer) that contains nickel (Ni) is formed onthe protective film 504 by a spin court method. Concerning the formationof the Ni containing layer, reference can be made to the abovepublication.

Thereafter, as shown in FIG. 7(B), heating processing at 570° C. for 14hours is performed in an inert atmosphere, and the amorphous siliconfilm 502 is crystallized. At this time, crystallization progressessubstantially in parallel with the substrate, starting from regions 506a and 506 b (hereinafter, designated as Ni addition region) with whichNi is in contact. As a result, a polysilicon film 507 is formed that hasa crystal structure in which bar crystals gather and form lines.

Thereafter, as shown in FIG. 7(C), an element (phosphorus preferably)that belongs to 15-family is added to the Ni addition regions 506 a and506 b, while leaving the protective film 504 as a mask. Regions 508 aand 508 b (hereinafter, designated as phosphorus addition region) towhich phosphorus was added at high concentration are thus formed.

Thereafter, heat processing at 600° C. for 12 hours is performed in aninert atmosphere as shown in FIG. 7(C). Ni existing in the polysiliconfilm 507 is moved by this heat processing, and almost all of them arefinally captured by the phosphorus addition regions 508 a and 508 b asshown by the arrow. It is thought that this is a phenomenon caused bythe gettering effect of a metallic element (Ni in this embodiment) byphosphorus.

By this process, the concentration of Ni remaining in the polysiliconfilm 509 is reduced to at least 2×10¹⁷ atoms/cm³ according to themeasurement value by SIMS (mass secondary ion analysis). Although Ni isa lifetime killer for a semiconductor, no adverse influence is given tothe TFT characteristic when it is decreased to this extent.Additionally, since this concentration is the measurement limit of theSIMS analysis in the current state of the art, it will show an evenlower concentration (less than 2×10¹⁷ atoms/cm³) in practice.

The polysilicon film 509 can be thus obtained that is crystallized by acatalyst and is decreased to the level in which the catalyst does notobstruct the operation of a TFT. Thereafter, active layers 510-513 thatuse the polysilicon film 509 only are formed by a patterning process. Atthis time, a marker to conduct mask alignment in the followingpatterning should be formed by using the above polysilicon film. (FIG.7(D))

Thereafter, a silicon nitride oxide film 50 nm thick is formed by theplasma CVD method as shown in FIG. 7(F), heating processing at 950° C.for 1 hour is then performed in an oxidation atmosphere, and a thermaloxidation process is performed. The oxidation atmosphere can be anoxygen atmosphere or another oxygen atmosphere in which halogen isadded.

In this thermal oxidation process, the oxidation progresses in theinterface between the active layer and the silicon nitride oxide film,and a polysilicon film whose thickness is about 15 nm is oxidized, sothat a silicon oxide film whose thickness is about 30 nm is formed. Thatis, a gate insulating film 514 of a thickness of 80 nm is formed inwhich the silicon oxide film 30 nm thick and the silicon nitride oxidefilm 50 nm thick are laminated. The film thickness of the active layers510-513 is made 30 nm by the thermal oxidation process.

Thereafter, as shown in FIG. 8(A), a resist mask 515 is formed, and animpurity element (hereinafter, designated as p-type impurity element)that gives the p-type through the medium of the gate insulating film 514is added. As the p-type impurity element, an element that belongs to13-family representatively, boron or gallium typically, can be used.This (called a channel dope process) is a process for controlling thethreshold voltage of a TFT.

In this embodiment, boron is added by the ion dope method in whichplasma is excitation is performed without the mass separation ofdiborane (B₂H₆). The ion implantation method that performs the massseparation can be used, of course. According to this process, impurityregions 516-518 are formed that include boron at the concentration of1×10¹⁵-1×10¹⁸ atoms/cm³ (5×10¹⁶-5×10¹⁷ atoms/cm³ representatively).

Thereafter, resist masks 519 a and 519 b are formed as shown in FIG.8(B), and an impurity element (hereinafter, designated as n-typeimpurity element) that gives the n-type through the medium of the gateinsulating film 514 is added. As the n-type impurity element, an elementthat belongs to 15-family representatively, phosphorus or arsenictypically, can be used. In this embodiment, a plasma doping method inwhich plasma excitation is performed without the mass separation ofphosphine (PH₃) is used. Phosphorus is added in the concentration of1×10¹⁸ atoms/cm³. The ion implantation method that performs massseparation can be used, of course.

A dose amount is adjusted so that the n-type impurity element isincluded in the n-type impurity regions 520, 521 formed by this processat the concentration of 2×10¹⁶-5×10¹⁹ atoms/cm³ (5×10¹⁷-5×10¹⁸ atoms/cm³representatively).

Thereafter, a process is performed for activating the added n-typeimpurity element and the added p-type impurity element as shown in FIG.8(C). There is no need to limit the activation means, but, since thegate insulating film 514 is disposed, the furnace annealing process thatuses an electro-thermal furnace is desirable. Additionally, it ispreferable to perform heat processing at a temperature as high aspossible because there is a possibility of having damaged the interfacebetween the active layer and the gate insulating film of a part that isa channel formation region in the process of FIG. 8(A).

Since the crystallization glass with high heat resistance is used inthis embodiment, the activating process is performed by the furnaceannealing processing at 800° C. for 1 hour. The thermal oxidation can beperformed keeping a processing atmosphere in an oxidizing atmosphere, orthe heat processing can be performed in an inert atmosphere.

This process clarifies the edge of the n-type impurity regions 520, 521,namely, the boundary (junction) between the n-type impurity regions 520,521 and the region (p-type impurity region formed by the process of FIG.8(A)) around the n-type impurity regions 520, 521, where the n-typeimpurity element is not added. This means that the LDD region and thechannel formation region can form an excellent junction when a TFT islater completed.

Thereafter, a conductive film 200-400 nm thick is formed, and patterningis performed, so that gate electrodes 522-525 are formed. The length ofeach TFT channel is decided by the line width of those gate electrodes522-525.

The gate electrode can be made of a conductive film of a single-layer,however, preferably, a lamination film, such as two-layer or three-layerfilm, is used when necessary. A known conductive film can be used as thematerial of the gale electrode. Specifically, use can be made of a filmof an element selected from the group consisting of aluminum (Al),tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W), chrome(Cr), and silicon (Si) having conductivity; a film of a nitride of theaforementioned elements (tantalum nitride film, tungsten nitride film,or titanium nitride film representatively); an alloy film of acombination of the aforementioned elements (Mo—W alloy or Mo—Ta alloyrepresentatively); or, a silicide film of the aforementioned elements(tungsten silicide film or titanium silicide film representatively).They can have a single-layer structure or a lamination-layer structure,of course.

In this embodiment, a lamination film is used that is made of a tungstennitride (WN) film 50 nm thick and a tungsten (W) film 350 nm thick. Thiscan be formed by the sputtering method. By adding an inert gas, such asXe or Ne, as a spattering gas, the film can be prevented from peelingoff because of stress.

At this time, the gate electrodes 523, 525 are formed to overlap withpart of the n-type impurity regions 520, 521, respectively, with thegate insulating film 514 therebetween. The overlapping part is latermade an LDD region overlapping with the gate electrode. According to thesectional view of the figure, the gate electrodes 524 a and 524 b areseen as separate, in fact, they are connected electrically to eachother.

Thereafter, with the gate electrodes 522-525 as masks, an n-typeimpurity element (phosphorus in this embodiment) is addedself-adjustably, as shown in FIG. 9(A). At this time, an adjustment isperformed so that phosphorus is added to the thus formed impurityregions 527-533 at the concentration of 2/1-1/10 (1/3-1/4representatively) of that of the n-type impurity regions 520, 521.Preferably, the concentration is 1×10¹⁶-5×10¹⁸ atoms/cm³ (3×10¹⁷-3×10¹⁸atoms/cm³ typically).

Thereafter, as shown in FIG. 9(B), resist masks 534 a-534 d are formedto cover the gate electrode, an n-type impurity element (phosphorus inthis embodiment) is then added, and impurity regions 535-541 including ahigh concentration of phosphorus are formed. The ion dope method usingphosphine (PH₃) is applied also herein, and an adjustment is performedso that the concentration of phosphorus in these regions is1×10²⁰-1×10²¹ atoms/cm³ (2×10²⁰-5×10²¹ atoms/cm³ representatively).

A source region or a drain region of the n-channel type TFT is formedthrough this process, and the switching TFT leaves a part of the n-typeimpurity regions 530-532 formed in the process of FIG. 9(A).

Thereafter, as shown in FIG. 9(C), the resist masks 534 a-534 d areremoved, and a resist mask 543 is newly formed. A p-type impurityelement (boron in this embodiment) is then added, and impurity regions544, 545 including a high concentration of boron are formed. Herein,according to the ion dope method using diborane (B₂H₆), boron is addedto obtain a concentration of 3×10²⁰-3×10²¹ atoms/cm³ (5×10²⁰-1×10²¹atoms/cm³ representatively).

Phosphorus has been already added to the impurity regions 544, 545 at aconcentration of 1×10²⁰-1×10²¹ atoms/cm³. Boron added herein has atleast three times as high concentration as the added phosphorus.Therefore, the impurity region of the n-type formed beforehand iscompletely changed into that of the p-type, and functions as an impurityregion of the p-type.

Thereafter, as shown in FIG. 9(D), the resist mask 543 is removed, andthen a first interlayer insulating film 546 is formed. As the firstinterlayer insulating film 546, an insulating film that includes siliconis used in the form of a single-layer structure or a stacked-layerstructure as a combination thereof. Preferably, the film thicknessthereof is 400 nm-1.5 μm. In this embodiment, a structure is created inwhich an 800 nm-thick silicon oxide film is stacked on a 200 nm-thicksilicon nitride oxide film.

Thereafter, the n-type or p-type impurity element added at eachconcentration is activated. The furnace annealing method is desirable asan activation means. In this embodiment heat treatment is performed at550° C. for 4 hours in a nitrogen atmosphere in an electro-thermalfurnace.

Heat treatment is further performed at 300-450° C. for 1-12 hours in anatmosphere that includes hydrogen of 3-100% for hydrogenation. This is aprocess to hydrogen-terminate an unpaired connector of a semiconductorfilm by thermally excited hydrogen. As another means for hydrogenation,plasma hydrogenation (hydrogen excited by plasma is used) can beperformed.

Hydrogenation can be performed during the formation of the firstinterlayer insulating film 546. In more detail, the 200 nm-thick siliconnitride oxide film is formed, and hydrogenation is performed asmentioned above, and thereafter the remaining 800 nm-thick silicon oxidefilm can be formed.

Thereafter, as shown in FIG. 10(A), contact holes are made in the firstinterlayer insulating film 546 and the gate insulating film 514, andsource wiring lines 547-550 and drain wiring lines 551-553 are formed.In this embodiment, this electrode is formed with a lamination film of athree-layer structure in which a 100 nm-thick Ti film, a 300 nm-thickaluminum film that includes Ti, and a 150 nm-thick Ti film arecontinuously formed according to the sputtering method. Other conductivefilms can be used, of course.

Thereafter, a first passivation film 554 is formed to be 50-500 nm thick(200-300 nm thick representatively). In this embodiment, a 300 nm-thicksilicon nitride oxide film is used as the first passivation film 554. Asilicon nitride film can be substituted for this.

At this time, it is effective to perform plasma treatment by the use ofgas that includes hydrogen, such as H₂ or NH₃, prior to the formation ofthe silicon nitride oxide film. Hydrogen excited by this preprocess issupplied to the first interlayer insulating film 546, and, through heattreatment, the film quality of the first passivation film 554 isimproved. At the same time, since hydrogen that is added to the firstinterlayer insulating film 546 diffuses onto the lower side, the activelayer can be effectively hydrogenated.

Thereafter, as shown in FIG. 10(B), a second interlayer insulating film555 made of organic resin is formed. Polyimide, acrylic fiber, or BCB(benzocyclobutene) can be used as the organic resin. Especially, sincethe second interlayer insulating film 555 is required to flatten thelevel differences formed by TFTs, an acrylic film excellent insmoothness is desirable. An acrylic film is formed to be 2.5 μm thick inthis embodiment.

Thereafter, contact holes that reach the drain wiring line 553 are madein the second interlayer insulating film 555 and the first passivationfilm 554, and then a protective electrode 556 is formed. As theprotective electrode 556, a conductive film largely composed of aluminumcan be used. The protective electrode 556 can be formed according to thevacuum deposition method.

Thereafter, an insulating film (a silicon oxide film in this embodiment)that includes silicon is formed to be 500 nm thick, an opening is thenformed at the position corresponding to the pixel electrode, and a thirdinterlayer insulating film 557 is formed. It is possible to easily forma tapered sidewall by using the wet etching method when the opening isformed. If the sidewall of the opening does not have a sufficientlygentle slope, deterioration of the EL layer caused by level differenceswill lead to an important problem.

Thereafter, a opposite electrode (MgAg electrode) 558 which is a cathodeof an EL element is formed. The MgAg electrode 558 is formed using thevacuum deposition method to make the film thickness 180-300 nm (200-250nm typically).

Next, an EL layer 559 is formed without air exposure by the vacuumdeposition method. The film thickness of the EL layer 559 is 800-200 nm(100-120 nm typically) and the pixel electrode (anode) 560 thereof is110 nm.

In this process, an EL layer and a pixel electrode (anode) aresequentially formed for a pixel corresponding to red, a pixelcorresponding to green, and a pixel corresponding to blue. However,since the EL layer is poor in tolerance to solutions, they must beindependently formed for each color without using the photolithographytechnique. Thus, it is preferable to conceal pixels except a desired oneby the use of the metal mask, and selectively form an EL layer and thepixel electrode (anode) for the desired pixel.

In detail, a mask is first set for concealing all pixels except a pixelcorresponding to red, and an EL layer and a pixel electrode (anode) ofred luminescence are selectively formed by the mask. Thereafter, a maskis set for concealing all pixels except a pixel corresponding to green,and an EL layer and a pixel electrode (anode) of green luminescence areselectively formed by the mask. Thereafter, as above, a mask is set forconcealing all pixels except a pixel corresponding to blue, and an ELlayer and a pixel electrode (anode) of blue luminescence are selectivelyformed by the mask. In this case, the different masks are used for therespective colors, Instead, the same mask may be used for them.Preferably, processing is performed without breaking the vacuum untilthe EL layer are formed for all the pixels.

A known material can be used for the EL layer 559. Preferably, that isan organic material in consideration of driving voltage. For example,the EL layer can be formed with a four-layer structure consisting of apositive hole injection layer, a positive hole transporting layer, aluminescent layer, and an electronic injection layer. In thisembodiment, indium tin oxide film is formed as a pixel electrode (anode)of an EL layer. A transparent conductive film can be used in which zincoxide (ZnO) of 2-20% is mixed with indium oxide or other well-knownmaterials also can be used.

At the final stage, a second passivation film 561 made of a siliconnitride film is formed to be 300 nm thick.

An EL display device constructed as shown in FIG. 10(C) is completed. Inpractice, preferably, the device is packaged (sealed) by a highlyairtight protective film (laminate film, ultraviolet cured resin film,etc.) or a housing material such as a ceramic sealing can, in order notto be exposed to the air when completed as shown in FIG. 10(C). In thatsituation, the reliability (life) of the EL layer is improved by makingthe inside of the housing material an inert atmosphere or by placing ahygroscopic material (for example, barium oxide) therein.

The present embodiment can be freely combined with an embodiment 2.

Embodiment 4

Now, the structure of the EL display device of this embodiment will bedescribed with reference to the perspective view of FIG. 11.

The EL display device of this embodiment is made up of a pixel portion2202, a gate signal side driving circuit 2203, and a source side drivingcircuit 2204, each formed on a glass substrate 2201. A switching TFT2205 of the pixel portion 2202 is an n-channel type TFT, and is disposedat the intersection of a gate wiring line 2206 connected to the gateside driving circuit 2203 and a source wiring line 2207 connected to thesource side driving circuit 2204. The drain region of the switching TFT2205 is connected to the gate electrode of the EL driver TFT 2208.

The source region of the EL driver TFT 2208 is connected to acurrent-feed line 2209. A capacitor 2216 is provided, which is connectedto the gate electrode and the current-feed line 2209 of the EL driverTFT 2208. In this embodiment, electric supply potential is added to thecurrent-feed line 2209. Further, a common potential is added to theopposite electrode (cathode in this embodiment) of the EL element 2211.

A FPC 2212 which is an external input-output terminal is provided withinput wiring lines (connection wiring lines) 2213, 2214 for transmittinga signal to the driving circuit, and an input-output wiring line 2214connected to the current-feed line 2209.

The EL module of this embodiment including housing materials will now bedescribed with reference to FIGS. 12(A) and 12(B). Reference charactersused in FIG. 11 are again used when necessary.

A pixel portion 2202, a gate signal side driving circuit 2203, and asource signal side driving circuit 2204 are formed on a glass substrate2201. Various wiring lines from each driving circuit are connected toexternal equipment via the input-output wiring lines 2213 to 2215 andthe FPC 2212.

At this time, a substrate 2304 is disposed so as to overlap at least thepixel portion 2202, preferably the pixel portion 2202 and the drivingcircuit 2203, 2204. The protective substrate 2304 is fixed to the glasssubstrate 2201 by a seal material 2305 so as to form closed space incooperation with the glass substrate 2201. At this time, the EL elementis in a state of being completely enclosed in the closed space, and iscompletely intercepted from the outside air. The closed space which isformed by a glass substrate 2201, a protective substrate 2304 and a sealmaterial 2305 is called a cell 2306. The plurality of cell 2306 can beformed by disposing a plurality of protective substrate 2304.

Preferably, the quality of the protective substrate 2304 is aninsulating substance such as glass or polymer. For example, there isamorphous glass (borosilicate glass, quartz, etc.), crystallizationglass, ceramics glass, organic resin (acrylic resin, styrene resin,polycarbonate resin, epoxy resin, etc.) or silicone resin. In addition,ceramics can be used. It is also possible to use metallic materials,such as stainless alloy, if the seal material 2305 is an insulatingmaterial.

As the quality of the seal material 2305, epoxy resin, acrylate resin,etc., can be used. In addition, thermosetting resin or light curingresin can be used as the adhesive. However, it is required to be amaterial that does not transmit oxygen and water to the utmost.

It is preferable to inject a packed material to the cell 2306. As apacked material, PVC (poly vinyl chloride), epoxy resin, silicon resin,PVB (poly vinyl butyral), EVA (ethylene vinyl acetate), acrylic andpolyimide etc. can be used.

It is also effective to dispose a drying agent in the cell 2306. A dryerdescribed in Japanese Laid-open Patent Publication Hei 9-148066 can beused as the drying agent. Generally, barium oxide can be used.

As shown in FIG. 12(B), the pixel portion is provided with a pluralityof pixels, each having individually isolated EL elements. All of themhave a protective electrode 2307 as a common electrode. In thisembodiment, a description was given as follows: it is preferable tocontinuously form the EL layer, the cathode (MgAg electrode), and theprotective electrode without air exposure. Instead, if the EL layer andthe cathode are formed by using the same mask material, and only theprotective electrode is formed by another mask, a structure of FIG.12(B) will be realized.

At this time, the EL layer and the cathode can be disposed on the pixelportion only, and are not required to be disposed on the drivingcircuit. No problem will occur even if they are disposed on the drivingcircuit, of course. However, they should not be disposed thereon inconsideration of the fact that an alkali metal is included in the ELlayer.

The protective electrode 2307 is connected to an input-output wiringline 2310 in the region shown by reference numeral 2308 through themedium of a connection wiring line 2309 that is made of the samematerial as the pixel electrode. The input-output wiring line 2310 is acurrent-feed line to give a EL driving potential to the protectiveelectrode 2307, and is connected to the FPC 2212 through the medium of aconductive paste material 2311.

Note that it is possible to freely combine the constitution of thisembodiment with the constitution of embodiment 1 and embodiment 2.

Embodiment 5

In Embodiment 5, there will be explained a structure of a pixel of an ELdisplay according to the present invention.

On the pixel portion of the EL display according to the presentinvention, a plurality of pixels are arranged in matrix. FIG. 13A showsan example of a circuit diagram of the pixel. In the pixel 1000, aswitching TFT 1001 is provided in FIG. 13A. Note that, in the presentinvention, as a switching TFT 1001, either an n-channel TFT or ap-channel TFT may be used. In FIG. 13A, the n-channel TFT is used as theswitching TFT 1001.

The gate electrode of the switching TFT 1001 is connected to the gatesignal line 1002 for inputting a gate signal. One of the source regionand the drain region of the switching TFT 1001 is connected to the datasignal line (also referred to as source signal line) 1003 for inputtinga digital data signal while the other is connected to the gate electrodeof the EL driver TFT 1004.

One of the source region and the drain region of the EL driver TFT 1004is connected to the power source supply line 1005 while the other isconnected to the EL element 1006.

The EL element 1006 comprises an anode, a cathode and an EL layerprovided between the anode and the cathode. Note that, according to thepresent invention, in the case where the anode is a pixel electrode andthe cathode is an opposite electrode, either the source region or thedrain region of the EL driver TFT 1004 is connected to the anode of theEL element 1006. On the contrary, in the case where the anode is theopposite electrode and the cathode is the pixel electrode, either thesource region or the drain region of the EL driver TFT is connected tothe cathode of the EL element 1006.

Note that as the EL driver TFT 1004, either n-channel TFT or p-channelTFT may be used. However, in the case where the anode of the EL element1006 is the pixel electrode and the cathode is the opposite electrode,it is preferable that the EL driver TFT 1004 is the p-channel TFT.Furthermore, on the contrary, in the case where the anode of the ELelement 1006 is the opposite electrode, and the cathode is the pixelelectrode, it is preferable that the EL driver TFT 1004 is an n-channelTFT. In FIG. 13A, the p-channel TFT is used as the EL driver TFT 1004.The cathode of the EL element 1006 is connected to the steady-statepower source 1007.

Furthermore, in the case where the switching TFT 1001 is set in thenon-selection state (Off state), a capacitor may be provided to hold thegate voltage of the EL driver TFT 1004. In the case where the capacitoris provided, the capacitor is connected between the one of the sourceregion and the drain region of the switching TFT 1001 which is notconnected to the source signal line, and the power source supply line1005. In a circuit diagram shown in FIG. 13A, the power source supplyline 1005 is arranged in parallel with the source signal line 1003.

Furthermore, an LDD region may be formed within the active layer of theEL driver TFT 1004, and a region may be formed (Lov region) in which theLDD region and the gate electrode overlap through the gate insulatingfilm. By forming the Lov region in the drain region side of the activelayer, a capacitance can be formed between the gate electrode of the ELdriver TFT 1004 and the Lov region, and the gate voltage of the ELdriver TFT 1004 can be stored, whether the EL driver TFT 1004 is ann-channel TFT or a p-channel TFT. In particular, when the EL driver TFT1004 is an n-channel TFT, the on current can be increased by forming theLov region in the drain region side of the active layer.

In order to use the Lov region of the EL driver TFT as a capacitor forstoring the gate voltage of the EL driver TFT 1004, a capacity value ofabout 19.8 fF is required in the case where the pixel size is 22 μm×22μm, the thickness of the gate insulating film is 800 Å and the relativedielectric constant of the gate insulating film is 4.1. Consequently, asthe area of the Lov region (an area in which the LDD region and the gateelectrode are overlapped via the gate insulating film), an area of about66 μm² is required.

Note that, in the circuit diagram shown in FIG. 13A, either theswitching TFT 001 or the EL driver TFT 1004 may be formed into amulti-gate structure (a structure including an active layer having twoor more channel forming regions connected in series). FIG. 14A shows acircuit diagram of a pixel in which the switching TFT 1001 of the pixelshown in FIG. 13A is formed into a multi-gate structure.

The switching TFT 1001 a and the switching TFT 1001 b are formedconnected in series. Except for the switching TFTs 1001 a and 1001 b,the structure is the same as the circuit diagram shown in FIG. 13A. Bymaking the switching TFTs multi-gate structures, the off current can belowered, and the gate voltage of the EL driver TFT 1004 can be storedwithout forming a capacitor, in particular. Therefore, a capacitor forstoring the gate voltage of the EL driver TFT 1004 need not be formed.Note that a double gate structure is used in FIG. 14A, but the presentinvention is not limited to a double gate structure, and any multi-gatestructure may be used.

Besides, though not shown, in the case where the EL driver TFT is formedinto a multi-gate structure, the deterioration of the EL driver TFT byheat can be suppressed.

Next, FIG. 13B shows another example of the circuit diagram of the pixelaccording to the present invention. In FIG. 13B, the switching TFT 1101is provided in the pixel 1100. Note that, in the present invention,either the n-channel TFT or the p-channel TFT may be used as theswitching TFT 1101. In FIG. 13B, the n-channel TFT is used as theswitching TFT 1101. The gate electrode of the switching TFT 1101 isconnected to the gate signal line 1102 for inputting the gate signal.One of the source region and the drain region of the switching TFT 1101is connected to the data signal line (also referred to as a sourcesignal line) 1103 for inputting a digital data signal while the other isconnected to the gate electrode of the EL driver TFT 1104.

Then, one of the source region and the drain region of the EL driver TFT1104 is connected to the power source supply line 1105 while the otheris connected to the EL element 1106.

The EL element 1106 comprises an anode, a cathode and an EL layerprovided between the anode and the cathode. Note that, in the presentinvention, in the case where the anode is the pixel electrode and thecathode is the opposite electrode, either the source region or the drainregion of the EL driver TFT 1104 is connected to the anode of the ELelement 1106. On the contrary, in the case where the anode is theopposite electrode and the cathode is the pixel electrode, either thesource region or the drain region of the EL driver TFT 1104 is connectedto the cathode of the EL element 1106. Note that, as the EL driver TFT1104, either the n-channel TFT or the p-channel TFT may be used.However, in the case where the anode of the EL element 1106 is the pixelelectrode and the cathode thereof is the opposite electrode, it ispreferable that the EL driver TFT 1104 is the p-channel TFT.Furthermore, on the contrary, in the case where the anode of the ELelement 1106 is the opposite electrode and the cathode thereof is thepixel electrode, it is preferable that the EL driver TFT 1104 is then-channel TFT. In FIG. 13B, the p-channel TFT is used in the EL driverTFT 1104. The cathode of the EL element 1106 is connected to thesteady-state power source 1107.

Furthermore, when the switching TFT 1101 is set in the non-selectionstate (Off state), a capacitor may be provided to hold the gate voltageof the EL driver TFT 1104. In the case where the capacitor is provided,the capacitor is connected between the one of the source region and thedrain region of the switching TFT 1101 which is not connected to thesource signal line, and the power source supply line 1105. In thecircuit diagram shown in FIG. 13B, the power source supply line 1105 andthe gate signal line 1102 are arranged in parallel.

Furthermore, an LDD region may be formed within the active layer of theEL driver TFT 1104, and a region may be formed (Lov region) in which theLDD region and the gate electrode overlap through the gate insulatingfilm. By forming the Lov region in the drain region side of the activelayer, a capacitance can be formed between the gate electrode of the ELdriver TFT 1004 and the Lov region, and the gate voltage of the ELdriver TFT 1004 can be stored, whether the EL driver TFT 1104 is ann-channel TFT or a p-channel TFT. In particular, when the EL driver TFT1104 is an n-channel TFT, the on current can be increased by forming theLov region in the drain region side of the active layer.

Note that, in the circuit diagram shown in FIG. 13B, either theswitching TFT 1101 or the EL driver TFT 1104 may be formed into amulti-gate structure. FIG. 14B shows a circuit diagram of a pixel inwhich the switching TFT 1101 of the pixel shown in FIG. 13B is formedinto a multi-gate structure.

The switching TFT 1101 a and the switching TFT 1101 b are formedconnected in series. Except for the switching TFTs 1101 a and 1101 b,the structure is the same as the circuit diagram shown in FIG. 13B. Bymaking the switching TFTs multi-gate structures, the off current can belowered, and the gate voltage of the EL driver TFT 1104 can be storedwithout forming a capacitor, in particular. Therefore, a capacitor forstoring the gate voltage of the EL driver TFT 1104 need not be formed.Note that a double gate structure is used in FIG. 14B, but the presentinvention is not limited to a double gate structure, and any multi-gatestructure may be used.

Besides, though not shown, in the case where the EL driver TFT is formedin a multi-gate structure, the deterioration of the EL driver TFT byheat can be suppressed.

Next, FIG. 15A shows another example of a circuit diagram of a pixelaccording to the present invention. In FIG. 15A, the pixel 1200 and thepixel 1210 are provided adjacent to each other. In FIG. 15A, referencenumerals 1201 and 1211 denote switching TFTs. Note that, in the presentinvention, as switching TFTs 1201 and 1211, either the n-channel TFT orthe p-channel TFT may be used. In FIG. 15A, the n-channel TFT is used inthe switching TFT 1201 and the switching TFT 1211. The gate electrodesof the switching TFTs 1201 and 1211 are connected to the gate signalline 1202 for inputting the gate signal. One of the source region andthe drain region of the switching TFTs 1201 and 1211 is connected to thedata signal lines 1203 and 1204 (hereinafter referred to as a sourcesignal line) for inputting a digital data signal while the other isconnected to the gate electrodes of the EL driver TFTs 1204 and 1214,respectively.

Then, one of the source region and the drain region of the EL driverTFTs 1204 and 1214 is connected to the power source supply line 1220while the other is connected to the EL elements 1205 and 1215,respectively. In this manner, in Embodiment 5, two adjacent pixels shareone power source supply line 1220. As a consequence, as compared withthe structure shown in FIG. 13 and FIG. 14, the number of the powersource supply lines can be decreased. When the ratio of the wiring withrespect to the whole pixel portion is small, the light shielding by thewiring can be suppressed in the case where the wiring is provided in adirection of the light emission of the EL layer.

The EL elements 1205 and 1215 comprise an anode, a cathode, and an ELlayer provided between the anode and the cathode respectively. Notethat, according to the present invention, in the case where the anode isthe pixel electrode and the cathode is the opposite electrode, eitherthe source region or the drain region of the EL driver TFTs 1204 and the1214 is connected to the anodes of the EL elements 1205 and 1215. On thecontrary, in the case where the anode is the opposite electrode and thecathode is the pixel electrode, either the source region or the drainregion of the EL driver TFTs 1204 and 1214 is connected to the cathodesof the EL elements 1205 and 1215. Note that, as the EL driver TFTs 1204and 1214, either the n-channel TFT or the p-channel TFT may be used.However, in the case where the anodes of the EL elements 1205 and 1215are pixel electrodes while the cathodes thereof are opposite electrodes,it is preferable that the EL driver TFTs 1204 and 1214 are the p-channelTFTs. Besides, on the contrary, in the case where the anodes of the ELelements 1205 and 1215 are the opposite electrodes and the cathodesthereof are the pixel electrodes, preferably the EL driver TFTs 1204 and1214 are n-channel TFTs. In FIG. 15A, as the EL driver TFTs 1204 and1214, the p-channel TFTs are used. The cathodes of the EL elements 1205and 1215 are connected to the steady-state power sources 1206 and 1216.

Furthermore, when the switching TFT 1201 and 1211 are set in thenon-selection state (Off state), a capacitor may be provided for storingthe gate voltage of the EL driver TFTs 1204 and 1214. In the case wherethe capacitor is provided, the capacitor may be connected between theone of the drain region and the source region of the switching TFT 1201which is not connected to the source signal line and the power sourcesupply line 1220.

Furthermore, an LDD region may be formed within the active layer of theEL driver TFTs 1204 and 1214, and a region may be formed (Lov region) inwhich the LDD region and the gate electrode overlap through the gateinsulating film. By forming the Lov region in the drain region side ofthe active layer, a capacitance can be formed between the gateelectrodes of the EL driver TFTs 1204 and 1214 and the Lov region, andthe gate voltages of the EL driver TFT 1204 and 1214 can be stored,whether the EL driver TFT 1204 is an n-channel TFT or a p-channel TFT.In particular, when the EL driver TFTs 1204 and 1214 are n-channel TFTs,the on current can be increased by forming the Lov region in the drainregion side of the active layer.

Note that, in a circuit diagram shown in FIG. 15A, the switching TFT1201 and 1211, or the EL driver TFTs 1204 and 1214 may be formed into amulti-gate structure. FIG. 16A shows a circuit diagram of a pixel inwhich the switching TFTs 1201 and 1211 are formed into the multi-gatestructure of a pixel shown in FIG. 15A.

The switching TFT 1201 a and the switching TFT 1201 b are formedconnected in series. The switching TFT 1211 a and the switching TFT 1211b are also formed connected in series. Except for the switching TFTs1201 a and 1201 b and the switching TFTs 1211 a and 1211 b, thestructure is the same as the circuit diagram shown in FIG. 15A. Bymaking the switching TFTs multi-gate structures, the off current can belowered, and the gate voltage of the EL driver TFTs 1204 and 1214 can bestored without forming a capacitor, in particular. Therefore, acapacitor for storing the gate voltages of the EL driver TFTs 1204 and1214 need not be formed. Note that a double gate structure is used inFIG. 16A, but the present invention is not limited to a double gatestructure, and any multi-gate structure may be used.

Besides, though not shown, in the case where the EL driver TFT is formedinto a multi-gate structure, the deterioration of the EL driver TFT byheat can be suppressed.

Next, FIG. 15B shows another example of a circuit diagram of a pixelaccording to the present invention. In FIG. 15B, the pixel 1300 and thepixel 1310 are provided adjacent to each other. In FIG. 151, referencenumerals 1301 and 1311 denote the switching TFTs. Note that, in thepresent invention, as the switching TFTs 1301 and 1311, either then-channel TFT or the p-channel TFT can be used. In FIG. 15B, then-channel TFT is used as the switching TFTs 1301 and 1311. The gateelectrodes of the switching TFTs 1301 and the 1311 are connected to thegate signal lines 1302 and 1312 for inputting the gate signalrespectively. One of the source region and the drain region of theswitching TFTs 1301 and 1311 is connected to the data signal line 1303(also referred to as a source signal line) for inputting a digital datasignal, while the other is connected to the gate electrode of the ELdriver TFTs 1304 and 1314 respectively.

Then, one of the source region and the drain region of the EL driverTFTs 1304 and 1314 is connected to the power source supply line 1320,while the other is connected to the EL elements 1305 and 1315respectively. In this manner, in Embodiment 5, two adjacent pixels shareone power source supply line 1320. As a consequence, as compared withthe structure shown in FIGS. 13 and 14, the number of power sourcesupply lines can be decreased. When the ratio of the wiring with respectto the whole pixel portion is small, the light shielding by the wiringcan be suppressed in the case where the wiring is provided in adirection of light emission of the EL layer. Then, in a circuit diagramshown in FIG. 16B, the power source supply line 1320 is provided inparallel with the gate signal lines 1302 and 1312.

The EL elements 1305 and 1315 comprise an anode, a cathode, and an ELlayer provided between the anode and the cathode respectively. Notethat, according to the present invention, in the case where the anode isthe pixel electrode and the cathode is an opposite electrode, either thesource region or the drain region of the EL driver TFTs 1304 and 1314 isconnected to the anodes of the EL elements 1305 and 1315. On thecontrary, in the case where the anode is the opposite electrode and thecathode is the pixel electrode, either the source region or the drainregion of the EL driver TFTs 1304 and 1314 is connected to the cathodesof the EL elements 1305 and 1315. Note that, as the EL driver TFTs 1304and 1314, either the n-channel TFT or the p-channel TFT may be used.However, in the case where the anodes of the EL elements 1305 and 1315are pixel electrodes and the cathodes thereof are opposite electrodes,it is preferable that the EL driver TFTs 1304 and 1314 are p-channelTFTs. Besides, on the contrary, in the case where the anodes of the ELelements 1305 and 1315 are opposite electrodes and the cathodes thereofare pixel electrodes, it is preferable that the EL driver TFTs 1304 and1314 are n-channel TFTs. In FIG. 15B, the p-channel TFTs are used as theEL driver TFTs 1304 and 1314, so that the cathodes of the EL elements1305 and 1315 are connected to the steady-state power sources 1306 and1316.

Furthermore, when the switching TFTs 1301 and 1311 are set in thenon-selection state (Off state), a capacitor may be provided for storingthe gate voltage of the EL driver TFTs 1304 and 1314. In the case wherethe capacitor is provided, the capacitor is connected between the one ofthe source region and the drain region of the switching TFTs 1301 and1311 which is not connected to the source signal line, and the powersource supply line 1320.

Furthermore, an LDD region may be formed within the active layer of theEL driver TFTs 1304 and 1314, and a region may be formed (Lov region) inwhich the LDD region and the gate electrode overlap through the gateinsulating film. By forming the Lov region in the drain region side ofthe active layer, a capacitance can be formed between the gateelectrodes of the EL driver TFT 1304 and 1314 and the Lov region, andthe gate voltage of the EL driver TFT 1304 and 1314 can be stored,whether the EL driver TFT 1304 and 1314 are n-channel TFTs or p-channelTFTs. In particular, when the EL driver TFT 1304 and 1314 are n-channelTFTs, the on current can be increased by forming the Lov region in thedrain region side of the active layer.

Note that, in a circuit diagram shown in FIG. 15B, the switching TFTs1301 and 1311 or the EL driver TFTs 1304 and 1314 may be formed into amulti-gate structure. FIG. 16B shows a circuit diagram of a pixel inwhich the switching TFTs 1301 and 1311 of a pixel shown in FIG. 15B areformed into a multi-gate structure.

The switching TFT 1301 a and the switching TFT 1301 b are formedconnected in series. The switching TFT 1311 a and the switching TFT 1311b are also formed connected in series. Except for the switching TFTs1301 a and 1301 b and the switching TFTs 1311 a and 1311 b, thestructure is the same as the circuit diagram shown in FIG. 15B. Bymaking the switching TFTs multi-gate structures, the off current can belowered, and the gate voltage of the EL driver TFTs 1304 and 1314 can bestored without forming a capacitor, in particular. Therefore, acapacitor for storing the gate voltage of the EL driver TFTs 1304 and1314 need not be formed. Note that a double gate structure is used inFIG. 16B, but Embodiment 5 is not limited to a double gate structure,and any multi-gate structure may be used.

Besides, though not shown, in the case where the EL driver TFT is formedinto the multi-gate structure, the deterioration of the EL driver TFTsby heat can be suppressed.

Note that, in Embodiment 5, a resistor may be provided between the pixelelectrodes the EL element and the drain region of the EL driver TFT. Byproviding the resistor, the quantity of current supplied from the ELdriver TFT to the EL element is controlled so that the influence of thecharacteristic of the EL driver TFT on the variation may be prevented.The resistor may be an element showing a resistance value sufficientlylarger than the on resistance of the EL driver TFT. Therefore, thestructure or the like is not restricted. Note that, the on resistance isa value obtained by dividing the drain voltage of the TFT by the draincurrent which flows at that time when the TFT is turned on. As aresistance value of the resistor, any in the range of 1 kΩ through 50 MΩ(preferably, 10 kΩ through 10 MΩ, or more preferably 50 kΩ through 1 MΩ)may be selected. When a semiconductor layer having a high resistancevalue as a resistor is used, the formation is easy and preferable.

Note that it is possible to freely combine Embodiment 5 with Embodiments1 to 3.

Embodiment 6

There are cases in which various compensations are necessary indisplaying an image by an electro-optical device. For example, there aregamma compensation and compensation by strengthening the light emittedby self emitting elements. Further, when handling a signal applied forgamma compensation in a CRT, there are times when it becomes necessaryfor reverse gamma compensation. In Embodiment 6, a compensation systemcapable of providing compensation for a digital video signal used in thepresent invention is explained.

An example of a compensation system which applies compensation to a 4bit digital video signal is explained below. Note that Embodiment 6 isnot limited to this number of bits. The compensation system used inEmbodiment 6 applies compensation to a video signal before it is inputto the time division gradation data signal generation circuit 114 shownin FIG. 4A. Note that it is necessary for the compensated video signalto be a digital signal, and therefore when the video signal is analog,it is first converted to digital and then the compensation is applied.

FIG. 20 is a graph showing a video signal before being input to thecompensation system (pre-compensation video signal) on the horizontalaxis, and showing the video signal after it is output from thecompensation system (post-compensation video signal) on the verticalaxis. When applying this type of compensation to the video signal,specifically the compensation system shown in FIGS. 21A and 21B isformed before the time division gradation data signal generator circuit.

Shown in FIG. 21A is one example of a compensation system used by thepresent invention. Non-volatile memories 901 to 904, the same number asthe number of bits in the video signal, are formed in the compensationsystem shown in FIG. 21A.

Information for each bit of the pre-compensation video signal is inputin order to in1 to in4. Note that the first bit (least sensitive bit,LSB) of the pre-compensation video signal is input to in1, and that thefourth bit (most sensitive bit, MSB) of the pre-compensation videosignal is input to in4.

The pre-compensation video signal having 4-bit information is input toall of the non-volatile memories 901 to 904.

A first bit of post-compensation video signal information for output,corresponding to the input pre-compensation video signal, is stored inthe non-volatile memory 901. Therefore, the pre-compensation videosignal input to the non-volatile memory 901 is converted to the firstbit of the post-compensation video signal and then output from out1.Note that the post-compensation video signal information for output,corresponding to the input pre-compensation video signal, is referred toas compensation data in the present invention.

Bits 2 to 4 of the post-compensation video signal information foroutput, corresponding to the input pre-compensation video signal, aresimilarly stored in the non-volatile memories 902 to 904. Therefore, thepre-compensation video signals input to the non-volatile memories 902 to904 are converted to the bits 2 to 4 of the post-compensation videosignal and then output from out2 to out4.

The specific state of the pre-compensation video signal having beentransformed to the post-compensation video signal is shown in FIG. 22.For a case in which the pre-compensation video signal input to in1 toin4 has (0000) information, information containing all 0s is output fromthe non-volatile memories 901 to 904. Therefore, the post-compensationvideo signal output from out1 to out4 contains (0000) information.

For a case in which the pre-compensation video signal input to in1 toin4 has (1000) information, 0 information is output from thenon-volatile memories 901, 903, and 904. Further, 1 information isoutput from the non-volatile memory 902. The post-compensation videosignal output from out1 to out4 therefore contains (0100) information.

In addition, for a case in which the pre-compensation video signal inputto in1 to in4 has (1111) information, information containing all 1s isoutput from the non-volatile memories 901 to 904. Therefore, thepost-compensation video signal output from out1 to out4 contains (1111)information.

A compensation such as that shown in FIG. 20 can thus be applied to thevideo signal by the compensation system using the non-volatile memories901 to 904.

Another example of a compensation system used by the present invention,different from that shown in FIG. 21A, is shown in FIG. 21B. Volatilememories 911 to 914, the same number as the number of bits in the videosignal, and storage use non-volatile memories 921 to 924 are formed inthe compensation system shown in FIG. 21B.

Information for each bit of the pre-compensation video signal is inputin order to in1 to in4. Note that the first bit (least sensitive bit,LSB) of the pre-compensation video signal is input to in1, and that thefourth bit (most sensitive bit, MSB) of the pre-compensation videosignal is input to in4.

The pre-compensation video signal having 4-bit information is input toall of the storage use non-volatile memories 921 to 924.

A first bit of post-compensation video signal information for output,corresponding to the input pre-compensation video signal, is stored inthe storage use non-volatile memory 921. Bits 2 to 4 of thepost-compensation video signal information for output, corresponding tothe input pre-compensation video signal, are similarly stored in thestorage use non-volatile memories 922 to 924. Information stored in thestorage use non-volatile memories 921 to 924 is then read into thevolatile memories 911 to 914, respectively, and stored for a fixedperiod.

The pre-compensation video signal input to the volatile memory 911 isthen converted into bit 1 of post-compensation video signal and outputfrom out1. Further, the pre-compensation video signals input to thevolatile memories 912 to 914 are also similarly converted to bits 2 to4, respectively, of the post-compensation video signal and output fromout2 to out4.

The pre-compensation video signal can thus be converted into thepost-compensation video signal in accordance with the compensationsystem shown in FIG. 21B. Note that it is possible to operate volatilememories at high speed, compared with non-volatile memories, andtherefore it is possible to operate the compensation system shown inFIG. 21B faster than the compensation system shown in FIG. 21A.

The compensation systems shown in FIGS. 21A and 21B have a memorycircuit, such as non-volatile memory, a volatile memory, or a storageuse non-volatile memory, divided into the same number as the number ofvideo signal bits, and these memory circuits may also be formed on thesame IC chip. Further, they also may be formed using semiconductors onthe same substrate as the EL display.

Note that the compensation data stored in the memory circuit in thecompensation system of Embodiment 6 is not limited to that used inEmbodiment 6.

Embodiment 7

The material used in the EL layer of the EL element in the EL display ofthe present invention is not limited to an organic EL material, and thepresent invention can be implemented using an inorganic EL material.However, at present inorganic EL materials have an extremely high drivervoltage, and therefore TFTs which have voltage resistancecharacteristics such that they are able to withstand such a high voltagemust be used.

Alternately, if an inorganic EL material having a lower driver voltageis developed in the future, it is possible to apply such a material tothe present invention.

Furthermore, it is possible to freely combine the constitution ofEmbodiment 7 with the constitution of any of Embodiments 1 to 6.

Embodiment 8

Organic materials used as an EL layer in the present invention may below molecular weight organic materials or polymer (high molecularweight) materials. Known low molecular weight organic materials centeraround the following: Alq₃ (tris-8-quinolinolate aluminum), and TPD(triphenylamine dielectric). A π-conjugate polymer material can be givenas an example of such a polymer organic material. Typically, materialssuch as PPV (polyphenylene vinylene), PVK (polyvinyl carbazole), orpolycarbonate can be given as examples.

Polymer (high molecular weight) materials can be formed by simple thinfilm formation methods such as spin coating (also referred to assolution application), dipping, dispensing, printing, and ink jetprinting, and they have a higher heat resistance compared to lowmolecular weight organic materials.

Further, in the EL elements of the EL display of the present invention,when the EL layers of the EL elements have an electron transportinglayer and a hole transporting layer, an electron transporting layer anda pole transporting layer may be composed of an inorganic material, forexample, of an amorphous semiconductor such as amorphous Si or amorphousSi_(1-x)C_(x).

A large amount of trap levels exist within amorphous semiconductors, anda large amount of boundary levels are formed in boundaries at whichother layers contact the amorphous semiconductors. Therefore, along withbeing able to emit light at a low voltage, the brightness of the ELelements can be made higher.

Further, a dopant (impurity) may also be added to an organic EL layer,changing the color of light emitted by the organic EL layer. Materialssuch as DCM1, nile red, ruberen, coumalin 6, TPB, and quinacridon can begiven as examples of dopants.

Furthermore, it is possible to freely combine the constitution ofEmbodiment 8 with the constitution of any of Embodiments 1 to 7.

Embodiment 9

Next, another method of driving the EL display of the present inventionshown in FIG. 1 is explained. A case of performing 2^(n) gray scaledisplay in accordance with an n-bit digital driver method is explained.Note that the timing chart is the same as for the case shown by theembodiment mode of the present invention, and therefore FIG. 2 may bereferenced.

The structure of a pixel portion 1500 of an EL display of the presentinvention is shown in FIG. 1. Gate signal lines (G1 to Gy) for inputtinggate signals are connected to gate electrodes of switching TFTs 1501 ofeach pixel. Further, one of source and drain regions of the switchingTFT 1501 of each pixel is connected to source signal lines (alsoreferred to as data signal lines) (S1 to Sx) for inputting digitalsignals, and the other is connected to gate electrode of EL driver TFT1504, and to a capacitor 1508 of each pixel. Note that this structure inthe embodiment mode has the capacitors 1508, but a structure notincluding the capacitors 1508 may also be used. There are no limitationsupon the present invention by the presence of, or lack of, capacitors.

One of the source and drain regions of the EL driver TFT 1504 of eachpixel is connected to power source supply lines (V1 to Vx), and theother is connected to EL elements 1506. The electric potential of thepower source supply lines (V1 to Vx) is referred to as an power sourceelectric potential. Further, the electric power source supply lines (V1to Vx) are connected to the capacitors 1508 of each pixel. Note that thedigital data signal is a signal in which an analog or a digital videosignal is transformed into a digital signal for performing time divisiongray scale display, and that it contains image information.

The EL element 1506 is composed of an anode, a cathode, and an EL layerformed between the anode and the cathode. When the anodes are connectedto the source regions or the drain regions of the EL driver TFTs 1504,namely when the anodes are the pixel electrodes, then the cathodes whichare opposite electrodes are maintained at a constant potential.Conversely, for a case in which the cathodes are connected to the sourceregions or the drain regions of the EL driver TFTs 1504, namely when thecathodes are the pixel electrodes, then the anodes which are theopposite electrodes are maintained at a constant potential.

The electric potential difference between the steady-state electricpotential of the opposite electrodes and the steady power sourceelectric potential of the pixel electrode is an EL driver voltage, andthe EL driver voltage is applied to the EL layers.

First, one frame period (F) is divided into n subframe periods (SF1 toSFn). Note that the period in which all of the pixels of the pixelportion display one image is referred to as one frame period (F).

The subframe periods are divided into address periods (Ta) and sustainperiods (Ts). The address period is the time required for inputting datainto all of the pixels during one subframe period, and the sustainperiod (also referred to as a turn on period) denotes a period in whichthe EL element is made to emit light.

The address periods (Ta) of SF1 to SFn become Ta1 to Tan, respectively.The sustain periods (Ts) of SF1 to SFn become Ts1 to Tsn, respectively.

In the beginning, in an address period, the power source supply lines(V1 through Vn) are held at the power source electric potential havingthe same height as the steady-state potential. In this specification,the power source electric potential in the digital driver address periodis referred to as off-power source electric potential. Note that, theheight of the off-power source electric potential may be set to the sameheight of the steady-state potential within the scope in which the ELelement 1506 does not emit light. Note that, the EL driver voltage atthis time is referred to as off EL driver voltage. Ideally, it isdesired that the off EL driver voltage is 0V, but may be set to a levelat which the EL element 1506 does not emit light.

Then, the gate signal is input to the gate signal line G1, so that allthe switching TFTs 1501 connected to the gate signal line G1 are allturned on.

With the switching TFT 1501 connected to the gate signal line G1 in theOn state, the digital data signal is input to the source signal lines(S1 to Sx) in order.

The digital data signal input into the source signal lines (S1 to Sx) isthen input to the gate electrodes of the EL driver TFTs 1504 through theswitching TFTs 1501, which are in the On state. Further, the digitaldata signal is also input to the capacitors 1508 of all of the pixelsconnected to the gate signal line G1, and an electric charge is stored.

Next, the gate signal is input to the gate signal line G2, so that allthe switching TFTs 1501 connected to the gate signal line G2 are turnedon. Then, in the state in which the switching TFTs 1501 connected to thegate signal line G2 are turned on, the digital data signal is input inorder to the source signal lines (S1 through Sx). The digital datasignal input to the source signal lines (S1 through Sn) is input to thegate electrode of the EL driver TFTs 1504 via the switching TFTs 1501.Furthermore, the digital data signal is also input, to the capacitor1508 of all pixels connected to gate signal line G2, and is held.

By repeating the above operations through to the gate signal line Gy,the digital data signal is input to all of the pixels. The period upuntil the digital data signal is input to all of the pixels is theaddress period.

The sustain period begins at the same time as the address period iscompleted. When the sustain period starts, the electric potential of theelectric power source supply lines (V1 to Vx) changes from an off-powersource electric potential to an on-power source electric potential. InEmbodiment 9, the power source electric potential during the digitaldrive sustain period is referred to as an on-power source electricpotential. The on-power source electric potential has an electricpotential difference between a level at which the EL elements emit lightand the steady-state electric potential. Note that this electricpotential difference is referred to as an on EL driver voltage. Notealso that the off-power source electric potential and the on-powersource electric potential are generically referred to as electric powersupply voltages. Further, the on EL driver voltage and an off EL drivervoltage are generically referred to as EL driver voltages.

All of the switching TFTs 1501 are set into the Off state in the sustainperiod. The digital data signal stored in the capacitors 1508 is theninput to the gate electrodes of the EL driver TFTs 1504.

When the digital data signal contains “0” information, then the ELdriver TFTs 1504 are set into the Off state in the embodiment mode ofthe present invention. The pixel electrodes of the EL elements 1506 aretherefore maintained at the off power source electric potential. As aresult, the EL elements 1506 contained in pixels to which the digitaldata signal containing “0” information is applied do not emit light.

On the other hand, for a case having “1” information in Embodiment 9,the EL driver TFTs 1504 turn on. The pixel electrodes of the EL elements1506 therefore have the on-power source electric potential. As a result,the EL elements 1506 having pixels to which the digital data signalhaving “1” information is applied emit light.

An address period again begins at the completion of the sustain period,and when the data signal is input to all of the pixels, a sustain periodbegins. All of the periods Ts1 to Ts(n−1) become the sustain period atthis point. The Ts(n−1) period turns on predetermined pixels here.

Hereinbelow, suppose that a similar operation is repeated with respectto the remaining n−2 sub-frames, so that the sustain periods Ts(n−2),Ts(n−3), . . . , Ts1 are set, and a predetermined pixel is lit inrespective sub-frames.

One frame period is complete after the n subframe periods are completed.

Note that, from among the n sustain periods Ts1, . . . , Tsn, thebrightness of light emitted by the EL elements during at least onesustain period is set to be always lower than the brightness of lightemitted by the EL elements in other sustain periods.

If a sustain period in which the brightness of light emitted is 1/m ofthat emitted in other sustain periods is taken as Tsp (where p is anarbitrary number greater than or equal to 1 and less than or equal ton), then from among the n sustain periods Ts1, . . . , Tsn, the lengthof the sustain periods other than the sustain period Tsp is expressed as2^(−(n−1)) T. Further, the length of the sustain period Tsp is expressedas 2^(−(p−1)) T×m. Note that m is a positive number greater than 1.Therefore, even if the brightness of light emitted by the EL elementsduring the sustain period Tsp is 1/m that of the light emitted duringthe other sustain periods, the length of the sustain period Tsp is setto be 2^(−(p−1)) T×m, and the predetermined gray scale display cantherefore be obtained.

Consequently, whichever of the n sustain periods Ts1, . . . , Tsn istaken as the sustain period Tsp, and however many sustain periods Tspare formed, if the amount of light emitted by the EL elements duringeach of the sustain periods Ts1, . . . Tsn is taken as Lm1, . . . , Lmn,then it becomes Lm1:Lm2:Lm3: . . . :Lm(n−1):Lmn=2⁰:2⁻¹:2⁻²: . . .:2^(−(n−2)):2^(−(n−1)). Note that SF1 to Sfn may appear in any order,and therefore the order of appearance of the sustain periods Ts1, . . ., Tsn is also arbitrary. By combining the sustain periods, the desiredgray scale display from among the 2^(n) gradations, can be performed.

The gradation of each pixel is determined by which subframe periods areselected for light emission during a one frame period. For example, ifn=8, and the brightness of pixels having light emitted during all of thesustain periods is taken as 100%, then for a case of the pixels emittinglight in Ts1 and Ts2, the brightness is expressed as 75%, and when Ts3,Ts5, and Ts8 are selected, the brightness can be expressed as 16%.

With the present invention, even if the Id-Vg characteristics of theTFTs vary slightly, dispersion in the amount of current output whenequal gate voltages are applied can be suppressed by the abovestructure. It therefore becomes possible to avoid the situation in whichthe amount of light emitted by EL elements of adjacent pixels differsgreatly due to variations in the Id-Vg characteristics, even with thesame voltage signal input.

Further, the amount of time light is emitted by the EL elements can beset to 2^(−(p−1))T×m (where T is a positive constant) in the sustainperiod Tsp, in which the brightness of light emitted by the EL elementsis 1/m of other sustain periods. By using the above structure, and byincreasing the number of image gradations, the number of bits n becomeslarger, and even if the length of the sustain period expressed by2^(−(n−1)) T becomes shorter, the brightness of light emitted by the ELelements is regulated to be 1/m that emitted in other sustain periods,the length of the sustain period is set to 2^(−(p−1)) T×m, and itbecomes possible to extend it.

Note that with the above structure of the present invention, theelectric potential of the opposite electrodes is always maintainedconstant, and the electric potential of the pixel electrodes is changedby the address periods and the sustain period, and the off EL drivervoltage or the on EL driver voltage is applied to the EL layers.However, the present invention is not limited to this structure.Alternatively, the electric potential of the pixel electrodes may bealways maintained constant, and by changing the electric potential ofthe opposite electrodes by the address period and the sustain period,the off EL driver voltage or the on EL driver voltage may be applied tothe EL layers. In this case, regulation of the brightness of the ELelements is performed by controlling the electric potential of theopposite electrodes.

Further, with the above structure of the present invention, the off ELdriver voltage is taken as zero and the EL elements are not made to emitlight, but the off EL driver voltage may also be set to the same voltageas the on EL driver voltage and light may also be emitted during theaddress period. In this case, the power source electric potential andthe steady-state electric potential are always maintained at a constantvalue. However, in this case the subframe period becomes a period ofemitting light, and therefore the length of the subframe periods is setto SF1, SF2, . . . , SFn=2⁰T, 2⁻¹T, . . . , 2^(−(n−1))T, and the lengthof the subframe period having a brightness of 1/m is set to2^(−(n−1))T×m. An image having a high brightness compared to that of adriver method in which light is not emitted during the address periodcan be obtained by the above structure.

In this embodiment, an example of driving by interlaced scanning hasbeen described, but it is also possible to drive by interlacing.

Furthermore, it is possible to combine the constitution of Embodiment 9with the constitutions of any of Embodiments 1, and 3 to 8.

Embodiment 10

A different method of driving the EL display of the present invention isexplained next. A case of performing 2⁴ gray scale display in accordancewith a 4-bit digital data signal is explained. Note that the timingchart is the same as for the case shown by Embodiment 2 of the presentinvention, and therefore FIG. 6 may be referenced.

The structure of a pixel portion of the EL display of Embodiment 10 isthe same as that shown in FIG. 1. The gate signal lines (G1 to Gy) forinputting gate signals are connected to a gate electrode of a switchingTFT in each pixel. Further, one set of regions from source regions anddrain regions of the switching TFTs of each pixel is connected to sourcesignal lines (S1 to Sx) for inputting digital data signals, and theother set of regions is connected to gate electrodes of EL driver TFTsof each pixel, and to capacitors of each pixel. Note that a structurehaving capacitors is used in Embodiment 10, but a structure not havingcapacitors may also be used. There are no limitations upon the presentinvention by the presence of, or lack of, capacitors.

One set of regions from source regions and drain regions of the ELdriver TFTs is connected to electric power source supply lines (V1 toVx), and the other set of regions is connected to EL elements. Theelectric potential of the electric power source supply lines (V1 to Vx)is referred to as an electric power source electric potential. Further,the electric power source supply lines (V1 to Vx) are connected to thecapacitors of each pixel.

The EL elements are composed of an anode and a cathode, and an EL layerformed between the anode and the cathode. When the anodes are connectedto the source regions or the drain regions of the EL driver TFTs, namelywhen the anodes are the pixel electrodes, then the cathodes of theopposed electrodes are held at a constant electric potential.Conversely, for a case in which the cathodes are connected to the sourceregions or the drain regions of the EL driver TFTs, namely when thecathodes are the pixel electrodes are held at a constant electricpotential, then the anodes which are the opposite electrodes.Furthermore, the electric potential of the opposite electrodes isreferred to as a steady-state electric potential within thisspecification.

The electric potential difference between the steady-state electricpotential of the opposite electrodes and the power source electricpotential of the pixel electrodes is an EL driver voltage, and the ELdriver voltage is applied to the EL layers.

FIG. 6 shows a timing chart with respect to digital driving of the ELdisplay of this embodiment. First, a one frame period (F) is dividedinto 4 pieces of subframe periods (SF1 to SF4).

The subframe periods are divided into address periods (Ta) and sustainperiods (Ts). The address period is the time required for inputtingdigital data signal into all of the pixels during one subframe period,and the sustain period (also referred to as a turn on period) denotes aperiod in which the EL element is made to emit light.

The address periods (Ta) having SF 1 to SF4 become Ta1 to Ta4,respectively. The sustain periods (Ts) having SF1 to SF4 become Ts1 toTs4, respectively.

In the beginning, in an address period, the power source supply lines(V1 through Vx) are held at the power source electric potential havingthe same height as the steady-state potential. In this specification,the power source electric potential in the digital driver address periodis referred to as off-power source electric potential. Note that, theheight of the off-power source electric potential may be set to the sameheight of the steady-state potential within the scope in which the ELelement does not emit light. Note that, the EL driver voltage at thistime is referred to as off EL driver voltage. Ideally, it is desiredthat the off EL driver voltage is 0V, but may be set to a level at whichthe EL element does not emit light.

Then, the gate signal is input to the gate signal line G1, so that allthe switching TFTs connected to the gate signal line G1 are all turnedon.

With the switching TFT connected to the gate signal line (G1 in the Onstate, the digital data signal is input to the source signal lines (S1to Sx) in order. The digital data signal has the information “0” and “1”and the digital data signals “0” and “1” will be signals where one willhave Hi voltage and the other will have Lo voltage.

The digital data signal input into the source signal lines (S1 to Sx) isthen input to the gate electrodes of the EL driver TFTs through theswitching TFTs, which are in the On state. Further, the digital datasignal is also input to the capacitors of all of the pixels connected tothe gate signal line G1, and an electric charge is stored.

Next, the gate signal is input to the gate signal line G2, so that allthe switching TFTs connected to the gate signal line G2 are turned on.Then, in the state in which the switching TFTs connected to the gatesignal line G2 are turned on, the digital signal is input in order tothe source signal lines (S1 through S4). The digital data signal inputto the source signal lines (S1 through Sx) is input to the gateelectrode of the EL driver TFTs via the switching TFTs. Furthermore, thedigital data signal is also input to the capacitor of all pixelsconnected to the gate signal line G2, and is held.

By repeating the above operations through the gate signal line Gy, thedigital data signal is input to all of the pixels. The period up untilthe digital data signal is input to all of the pixels is the addressperiod.

The sustain period begins at the same time as the address period iscompleted. When the sustain period starts, the electric potential of theelectric power source supply lines (V1 to Vx) changes from an off-powersource electric potential to an on-power source electric potential. Inthis embodiment, the power source electric potential during the digitaldrive sustain period is referred to as an on-power source electricpotential. The on-power source electric potential has an electricpotential difference between a level at which the EL elements emit lightand the steady-state electric potential. Note that this electricpotential difference is referred to as an on EL driver voltage. Notealso that the off-power source electric potential and the on-powersource electric potential are generically referred to as electric powersupply voltages. Further, the on EL driver voltage and an off EL drivervoltage are generically referred to as EL driver voltages.

All of the switching TFTs are set into the Off state in the sustainperiod. The digital data signal stored in the capacitors is then inputto the gate electrodes of the EL driver TFTs.

When the digital data signal contains “0” information, then the ELdriver TFTs are set into the Off state in the embodiment of the presentinvention. The pixel electrodes of the EL elements are thereforemaintained at the off power source electric potential. As a result, theEL elements 1506 having pixels to which the digital data signalcontaining “0” information is applied do not emit light.

On the other hand, for a case having “1” information in this embodimentthe EL driver TFTs turn on. The pixel electrodes of the EL elementstherefore have the on-power source electric potential. As a result, theEL elements having pixels to which the digital data signal having “1”information is applied emit light.

An address period again begins at the completion of the sustain period,and when the data signal is input to any one of the pixels, a sustainperiod begins. All of the periods Ts1 to Ts3 become the sustain periodat this point. The Ts3 period turns on predetermined pixels here.

Similar operations are repeated subsequently for the remaining 2subframe periods, Ts2, Ts1 are set in order as sustain periods, andpredetermined pixels are turned on in the respective subframes.

One frame period is complete after the 4 subframe periods are completed.

Note that, from among the four sustain periods Ts1, . . . Ts4, thebrightness of light emitted by the EL elements during at least onesustain period is set to be always lower than the brightness of lightemitted by the EL elements in other sustain periods. In this embodiment,the brightness of light emitted by the EL elements during the sustainperiods Ts3 and Ts4 is one-half the brightness of light emitted by theEL elements during the sustain periods Ts1 and Ts2. In other words, theon EL driver voltage during the sustain periods Ts3 and Ts4 is one-halfthe EL driver voltage during the other sustain periods Ts1 and Ts2.

The length of the sustain periods Ts1 and Ts2, the sustain periods otherthan the sustain periods Ts3 and Ts4 having an emitted light brightnessof one-half that of other sustain periods is expressed as T and 2⁻¹ T,respectively. Further, the length of the sustain periods Ts3 and Ts4 isexpressed as 2⁻² T×2 and 2⁻³ T×2, respectively. Namely, the ratio oflengths of the sustain periods Ts1 to Ts4 is 1:2⁻¹:2⁻¹:2⁻². Therefore,even if the brightness of light emitted by the EL elements during thesustain periods Ts3 and Ts4 is one-half that of the light emitted duringthe other sustain periods, the ratio of lengths of the sustain periodsTs3 and Ts4 to all of the sustain periods, is twice that of a case inwhich the brightness of light emitted is not set to one-half. Therefore,even though the brightness of light emitted by the EL elements in thesustain periods Ts3 and Ts4 is one half that of the other sustainperiods, the length ratio of the sustain periods Ts3 and Ts4 is set totwice as long, and the desired gray scale display can be obtained.

Consequently, in this embodiment, though the light brightness of the ELelement in the sustain periods Ts3 and Ts4 is set to one-half, whicheverof the 4 pieces of sustain periods Ts1, . . . , Ts4 is given a reducedbrightness, and however much the brightness is reduced, and however manylow brightness sustain periods are formed, if the amount of lightemitted by the EL elements during each of the sustain periods Ts1, . . .Ts4 is taken as Lm1, . . . , Lm4, then it becomes Lm1: . . .:Lm4=2⁰:2⁻¹:2⁻²:2⁻³. Note that SF1 to SF4 may appear in any order, andtherefore the order of appearance of the sustain periods Ts1, . . . ,Ts4 is also arbitrary. By combining the sustain periods, the desiredgray scale display, from among the 2⁴ gradations, can be performed.

The gradation of each pixel is determined by which subframe periods areselected for light emission during one frame period. For example, ifn=4, and the brightness of pixels having light emitted during all of thesustain periods is taken as 100%, then for a case of the pixels emittinglight in Ts1 and Ts2, the brightness is expressed as 80%, and when Ts2,Ts3, and Ts4 are selected, the brightness can be expressed as 47%.

With the present invention, even if the Id-Vg characteristics of theTFTs vary slightly, dispersion in the amount of current output whenequal gate voltages are applied can be suppressed by the abovestructure. It therefore becomes possible to avoid the situation in whichthe amount of light emitted by EL elements of adjacent pixels differsgreatly due to variations in the Id-Vg characteristics, even with thesame voltage signal input.

Further, in a sustain period Tsp in which the brightness of lightemitted is 1/m of that emitted in other sustain periods Ts1 to Tsn, ifthe lengths of the other sustain periods Ts1 to Tsn is taken as2^(−(n−1)) T (where T is a positive constant), then the light emittingtime of the El elements can be set as 2^(−(p−1)) T×m. In accordance withthe above structure, by regulating the brightness of light emitted bythe EL elements during the sustain period Tsp to be 1/m that emittedduring the other sustain periods, the length ratio of the sustain periodTsp to all of the sustain periods can be extended by a multiple of mcompared to a case in which the brightness of light emitted during thesustain period Tsp is not set to 1/m. Therefore, in accordance withincreasing the number of gray scale of images, even if the number ofbits n becomes larger and the length of the sustain periods becomesshorter, the length of the sustain periods can be extended by loweringthe brightness of the light emitted by the EL elements.

Further, an example of driving by non-interlaced scanning is explainedin this embodiment, but it is also possible to drive by interlacing.

Furthermore, it is possible to combine the constitution of Embodiment 10with the constitutions of any of Embodiments 1, and 3 to 8.

Embodiment 11

The EL display device (EL module) formed by performing the presentinvention is superior to a liquid crystal display device in visibilityin bright places because of its self-luminous properties. Therefore, thepresent invention can be used as a display portion of a direct-view typeEL display (indicating a display equipped with an EL module). As the ELdisplay, there are a personal computer monitor, a TV receiving monitor,an advertisement display monitor, and so on.

The present invention can be operated to all electronic equipment thatincludes displays as constituent parts, including the aforementioned ELdisplay.

As the electronic equipment, there are an EL display, video camera,digital camera, head mounted type display, car-navigator, personalcomputer, portable information terminal (mobile computer, mobile phone,electronic book, etc.), and picture reproducer provided with recordingmedia (specifically, device which can reproduce a recording medium andequip a display capable of displaying the image such as compact disk(CD), laser disc (LD), or digital video disc (DVD)). Examples of theelectronic equipment are shown in FIG. 17.

FIG. 17(A) depicts a personal computer, which includes a main body 2001,case 2002, display portion 2003, and keyboard 2004. The presentinvention can be used as the display device 2003.

FIG. 17(B) depicts a video camera, which includes a main body 2101,display device 2102, voice inputting portion 2103, operation switch2104, battery 2105, and image reception portion 2106. The presentinvention can be used as the display device 2102.

FIG. 17(C) depicts a part of a head mounted type EL display (rightside), which includes a main body 2301, signal cable 2302, head fixationband 2303, display monitor 2304, optical system 2305, and display device2306. The present invention can be used as the display device 2306.

FIG. 17(D) depicts a picture reproducer (specifically, DVD reproducingplayer) provided with recording media, which includes a main body 2401,recording medium 2402 (CD, LD, DVD, etc.), operation switch 2403,display device (a) 2404, and display panel (b) 2405. The display device(a) 2404 chiefly displays image information, and the display device (b)2405 chiefly displays character information. The present invention canbe used as the display devices (a) 2404 and (b) 2405. The presentinvention is applicable to a CD player or a game machine as a picturereproducer provided with recording media.

FIG. 17(E) depicts a portable (mobile) computer, which includes a mainbody 2501, camera portion 2502, image reception part 2503, operationswitch 2504, and display portion 2505. The present invention can be usedas the display device 2505.

If the luminescence brightness of the EL material is enhanced in thefuture, the present invention will be applicable to a front or rear typeprojector.

The present invention has a quite wide scope of application, asmentioned above, and is applicable to electronic equipment in allfields. The electronic equipment of this embodiment can be realized bythe using any structure resulting from the free combination ofembodiments 1 to 10.

With the above structure of the present invention, it becomes possibleto regulate the light emission brightness of an EL element in accordancewith the value of an EL driver voltage applied to the EL element, and itbecomes possible to display a vivid image with good balance between thebrightnesses of red color, blue color, and green color light emissions.In addition, even if the amount of current controlled by an EL driverTFT increases due to the applied voltages becoming larger, deteriorationof the EL driver TFT can be suppressed.

With the present invention, even if the Id-Vg characteristics of theTFTs vary slightly, dispersion in the amount of current output whenequal gate voltages are applied can be suppressed by the abovestructure. It therefore becomes possible to avoid the situation in whichthe amount of light emitted by EL elements of adjacent pixels differsgreatly due to variations in the Id-Vg characteristics, even with thesame voltage signal input.

Further, in a sustain period Tsp in which the brightness of light of Elelement emitted is 1/m of that emitted in other sustain periods Ts1 toTsn, if the lengths of the other sustain periods Ts1 to Tsn is taken as2^(−(n−1)) T (where T is a positive constant), then the light emittingtime of the EL element can be set as 2^(−(p−1)) T×m. In accordance withthe above structure, by regulating the brightness of light emitted bythe EL elements during the sustain period Tsp to be 1/m that emittedduring the other sustain periods, the length ratio of the sustain periodTsp to all of the sustain periods can be extended by a multiple of mcompared to a case in which the brightness of light emitted during thesustain period Tsp is not set to 1/m. Therefore, in accordance withincreasing the number of display gradations, even if the number of bitsn becomes larger and the length of the sustain periods becomes shorter,the length of the sustain periods can be extended by lowering thebrightness of the light emitted by the EL elements.

1. A light emitting device comprising: at least a thin film transistorcomprising a semiconductor layer, a gate electrode and a gate insulatingfilm between the semiconductor layer and the gate electrode, over aninsulating surface; a first insulating film over the thin filmtransistor; a second insulating film over the first insulating film; afirst electrode formed in a contact hole formed in the first and secondinsulating films, and electrically connected to the thin film transistorthrough the contact hole; a third insulating film over the secondinsulating film and covering an edge portion of the first electrode; anEL layer over the first electrode; and a second electrode over the ELlayer, wherein an edge portion of the third insulating film has a tapershape.
 2. A light emitting device according to claim 1, wherein thefirst insulating film comprising a material selected from the groupconsisting of silicon oxide, silicon nitride oxide and silicon nitride.3. A light emitting device according to claim 1, wherein the secondinsulating film comprising an organic resin selected from the groupconsisting of polyimide, polyamide, acrylic resin and BCB(benzocyclobutene).
 4. A light emitting device according to claim 1,wherein the third insulating film comprising a material selected fromsilicon oxide, silicon nitride oxide, and organic resin.
 5. A lightemitting device according to claim 1, wherein an angle of the tapershape is 10-60°.
 6. A light emitting device according to claim 1,wherein the light emitting device is applied to an electronic equipmentselected from the group consisting of a personal computer, a videocamera, a head mounted type EL display, a DVD and a portable computer.7. A light emitting device comprising: at least a thin film transistorcomprising a semiconductor layer, a gate electrode and a gate insulatingfilm between the semiconductor layer and the gate electrode, over aninsulating surface; a first insulating film over the thin filmtransistor; a second insulating film over the first insulating film; afirst electrode formed in a contact hole formed in the first and secondinsulating films, and electrically connected to the thin film transistorthrough the contact hole; a third insulating film over the secondinsulating film and covering an edge portion of the first electrode; anEL layer over the first electrode; and a second electrode over the ELlayer, wherein an edge portion of the third insulating film has a tapershape, and wherein the semiconductor layer comprises a LDD region, andthe LDD region and the gate electrode overlap through the gateinsulating film.
 8. A light emitting device according to claim 7,wherein the first insulating film comprising a material selected fromthe group consisting of silicon oxide, silicon nitride oxide and siliconnitride.
 9. A light emitting device according to claim 7, wherein thesecond insulating film comprising an organic resin selected from thegroup consisting of polyimide, polyamide, acrylic resin and BCB(benzocyclobutene).
 10. A light emitting device according to claim 7,wherein the third insulating film comprising a material selected fromsilicon oxide, silicon nitride oxide, and organic resin.
 11. A lightemitting device according to claim 7, wherein an angle of the tapershape is 10-60°.
 12. A light emitting device according to claim 7,wherein the light emitting device is applied to an electronic equipmentselected from the group consisting of a personal computer, a videocamera, a head mounted type EL display, a DVD and a portable computer.13. A light emitting device comprising: a first thin film transistorover an insulating surface, and comprising a semiconductor layer, firstand second gate electrodes, and a gate insulating film between thesemiconductor layer and the first and second gate electrodes; a secondthin film transistor over the insulating surface, and comprising asemiconductor layer, a gate electrode, and a gate insulating filmbetween the semiconductor layer and the gate electrode; a firstinsulating film over the first thin film transistor and the second thinfilm transistor; a first electrode electrically connected to the secondthin film transistor through a contact hole formed in the firstinsulating film; a second insulating film over the first insulating filmand covering an edge portion of the first electrode; an EL layer overthe first electrode; and a second electrode over the EL layer, whereinan edge portion of the second insulating film has a taper shape, andwherein the semiconductor layer of the second thin film transistorcomprises a LDD region, and the LDD region and the gate electrodeoverlap through the gate insulating film.
 14. A light emitting deviceaccording to claim 13, wherein the first insulating film comprising amaterial selected from the group consisting of silicon oxide, siliconnitride oxide and silicon nitride.
 15. A light emitting device accordingto claim 13, wherein the second insulating film comprising a materialselected from silicon oxide, silicon nitride oxide, and organic resin.16. A light emitting device according to claim 13, wherein an angle ofthe taper shape is 10-60°.
 17. A light emitting device according toclaim 13, wherein the light emitting device is applied to an electronicequipment selected from the group consisting of a personal computer, avideo camera, a head mounted type EL display, a DVD and a portablecomputer.
 18. A light emitting device comprising: a first thin filmtransistor over an insulating surface; a second thin film transistorover the insulating surface, and comprising a semiconductor layer, agate electrode, and a gate insulating film between the semiconductorlayer and the gate electrode; a first insulating film over the firstthin film transistor and the second thin film transistor; a firstelectrode electrically connected to the second thin film transistorthrough a contact hole formed in the first insulating film; a secondinsulating film over the first insulating film and covering an edgeportion of the first electrode; an EL layer over the first electrode;and a second electrode over the EL layer, wherein an edge portion of thesecond insulating film has a taper shape, and wherein the secondinsulating film overlaps with the first thin film transistor, and doesnot overlap with the second thin film transistor.
 19. A light emittingdevice according to claim 18, wherein the first insulating filmcomprising a material selected from the group consisting of siliconoxide, silicon nitride oxide and silicon nitride.
 20. A light emittingdevice according to claim 18, wherein the second insulating filmcomprising a material selected from silicon oxide, silicon nitrideoxide, and organic resin.
 21. A light emitting device according to claim18, wherein an angle of the taper shape is 10-60°.
 22. A light emittingdevice according to claim 18, wherein the light emitting device isapplied to an electronic equipment selected from the group consisting ofa personal computer, a video camera, a head mounted type EL display, aDVD and a portable computer.